What Is an OBS in Project Management? Structure Explained

<div class="ge-article-wrapper"><nav class="ge-toc" aria-label="Table of contents"><p class="ge-toc-label">In this article</p><ul class="ge-toc-list"><li><a href="#the-structure">The Structure</a></li><li><a href="#where-obs-meets-wbs-control-accounts">Where OBS Meets WBS: Control Accounts</a></li><li><a href="#worked-example-building-an-obs-for-a-35m-hospital-project">Worked Example: Building an OBS for a £35M Hospital Project</a></li><li><a href="#why-obs-matters-for-evm">Why OBS Matters for EVM</a></li><li><a href="#common-mistakes">Common Mistakes</a></li><li><a href="#frequently-asked-questions">Frequently Asked Questions</a></li></ul></nav><article class="ge-article-body"><p>An Organisational Breakdown Structure (OBS) is a hierarchical chart showing who's responsible for what on your project. It maps every person (or role) in the project team from the project director down to the site supervisors. On its own, it's useful. But the real power of the OBS comes when you overlay it on the <a href="/en/earned-value/definitions/budget-at-completion">Work Breakdown Structure</a>, because the intersection of "what work" and "who's responsible" creates your control accounts, and control accounts are the building blocks of <a href="/en/earned-value">earned value management</a>. </p><p>This term is part of the <a href="/en/earned-value/definitions">earned value definitions glossary</a>. For understanding the role that manages each control account, see <a href="/en/earned-value/definitions/control-account-manager">Control Account Manager (CAM)</a>. </p><h2 id="the-structure">The Structure</h2><p>The OBS looks like an org chart because it essentially is one, but oriented around the project, not the company. A company org chart shows reporting lines across the business. A project OBS shows reporting lines within a single project. On a £35M hospital refurbishment, the OBS doesn't care that the M&amp;E package manager also reports to a regional M&amp;E director. It only cares that, on this project, they're responsible for the M&amp;E control accounts. </p><pre class="ge-ascii-diagram ge-anim"> PROJECT DIRECTOR (Sarah Chen) | +---------------+---------------+ | | | PROJECT MANAGER COMMERCIAL LEAD DESIGN MANAGER (Mike Roberts) (Jane Walsh) (Tom Patel) | | | +---------+---------+ | +-----+-----+ | | | | | | PACKAGE PACKAGE PACKAGE | ARCHITECT STRUCT. MGR 1 MGR 2 MGR 3 | (External) ENGINEER (Struct) (M&amp;E) (Ext) | (External) (D.Brown) (K.Lee) (P.Jones) | | | +---+ +---+ +---+ | | | | | | S1 S2 S3 S4 S5 S6 &lt;-- Site Supervisors (Foremen and gangers on site) LEVEL 1: Project Director = Strategic oversight LEVEL 2: PM / Commercial = Delivery management LEVEL 3: Package Managers = Control account owners (CAMs) LEVEL 4: Site Supervisors = Work package execution </pre><p>Each level has a distinct purpose in EVM. Level 1 receives exception reports. Level 2 reviews all <a href="/en/earned-value/definitions/cost-performance-index">CPI</a> and <a href="/en/earned-value/definitions/schedule-performance-index">SPI</a> data. Level 3 generates the data. Level 4 provides the physical progress inputs that feed the calculations. </p><h2 id="where-obs-meets-wbs-control-accounts">Where OBS Meets WBS: Control Accounts</h2><p>This is the bit that matters for earned value. The WBS breaks the project into deliverables (what). The OBS breaks the project into responsibilities (who). Where they intersect, you get a control account, a defined chunk of work with a single owner, a budget, and a schedule baseline. </p><pre class="ge-ascii-diagram ge-anim"> WBS (What) ────────────────────── │ Structure │ M&amp;E │ External │ │ │ Works ──────────────────────┼──────────┼────────┼────────── OBS D. Brown │ CA-01 │ │ (Who) (Struct) │ £8.2M │ │ ──────────────────┼──────────┼────────┼────────── K. Lee │ │ CA-02 │ (M&amp;E) │ │ £12.4M │ ──────────────────┼──────────┼────────┼────────── P. Jones │ │ │ CA-03 (External) │ │ │ £6.1M ──────────────────┼──────────┼────────┼────────── CA = Control Account Each cell = one person responsible for one chunk of work Each CA has its own BAC, schedule baseline, and EVM metrics RULE: Every control account has exactly ONE owner. No shared responsibility. No committee decisions. </pre><p>That last rule is non-negotiable. The moment two people share responsibility for a control account, nobody is accountable. I've seen a £45M data centre project where they tried joint ownership of the M&amp;E control account between the electrical and mechanical leads. By month 6, both were blaming each other for a -£340K variance. Nobody owned the corrective action plan. The project director had to restructure the OBS mid-project, splitting it into separate electrical and mechanical control accounts. It worked, but it cost them two months of clean EVM data. </p><p>One person. One control account. Always. </p><h2 id="worked-example-building-an-obs-for-a-35m-hospital-project">Worked Example: Building an OBS for a £35M Hospital Project</h2><span class="ge-worked-label">Worked Example</span><div class="ge-callout ge-anim"><p><strong>Scenario:</strong> A £35M NEC4 Option A hospital refurbishment in Birmingham. The project has 8 control accounts across 4 packages. The contractor needs to build an OBS that maps to the WBS and creates clear accountability for <a href="/en/earned-value">earned value</a> reporting.</p><br><p><strong>Step 1: Define the WBS packages</strong></p><br><div class="ge-table-wrap ge-anim"><table class="ge-table"><thead><tr><th>WBS Code</th><th>Package</th><th><a href="/en/earned-value/definitions/budget-at-completion">BAC</a></th></tr></thead><tbody><tr><td>WBS-100</td><td>Structural &amp; Demolition</td><td>£8,200,000</td></tr><tr><td>WBS-200</td><td>M&amp;E Installation</td><td>£12,400,000</td></tr><tr><td>WBS-300</td><td>External Works &amp; Landscaping</td><td>£6,100,000</td></tr><tr><td>WBS-400</td><td>Fit-Out &amp; Finishes</td><td>£5,800,000</td></tr><tr><td>–</td><td>Prelims &amp; Management</td><td>£2,500,000</td></tr><tr><td><strong>Total</strong></td><td></td><td><strong>£35,000,000</strong></td></tr></tbody></table></div><br><p><strong>Step 2: Define the OBS roles</strong></p><br><p>- Project Director: Sarah Chen</p><p>- Project Manager: Mike Roberts</p><p>- Commercial Lead: Jane Walsh</p><p>- Package Manager (Structural): David Brown</p><p>- Package Manager (M&amp;E): Karen Lee</p><p>- Package Manager (External): Peter Jones</p><p>- Package Manager (Fit-Out): Rachel Adams</p><br><p><strong>Step 3: Map OBS to WBS, create control accounts</strong></p><br><div class="ge-table-wrap ge-anim"><table class="ge-table"><thead><tr><th>Control Account</th><th>WBS</th><th>OBS (CAM)</th><th>BAC</th><th>Report To</th></tr></thead><tbody><tr><td>CA-100</td><td>Structural &amp; Demo</td><td>D. Brown</td><td>£8.2M</td><td>M. Roberts</td></tr><tr><td>CA-200A</td><td>M&amp;E (Electrical)</td><td>K. Lee</td><td>£6.8M</td><td>M. Roberts</td></tr><tr><td>CA-200B</td><td>M&amp;E (Mechanical)</td><td>K. Lee</td><td>£5.6M</td><td>M. Roberts</td></tr><tr><td>CA-300A</td><td>External (Civils)</td><td>P. Jones</td><td>£3.4M</td><td>M. Roberts</td></tr><tr><td>CA-300B</td><td>External (market)</td><td>P. Jones</td><td>£2.7M</td><td>M. Roberts</td></tr><tr><td>CA-400A</td><td>Fit-Out (Partitions &amp; Ceilings)</td><td>R. Adams</td><td>£3.1M</td><td>M. Roberts</td></tr><tr><td>CA-400B</td><td>Fit-Out (Decorations &amp; Flooring)</td><td>R. Adams</td><td>£2.7M</td><td>M. Roberts</td></tr><tr><td>CA-500</td><td>Prelims &amp; Management</td><td>M. Roberts</td><td>£2.5M</td><td>S. Chen</td></tr></tbody></table></div><br><p>Karen Lee owns two control accounts (CA-200A and CA-200B). That's fine, the same person can own multiple control accounts. What matters is that each control account has only one owner. Karen reports CPI and SPI separately for electrical and mechanical, and the commercial lead consolidates them for the monthly earned value report.</p><br><p><strong>Month 5 results (partial):</strong></p><br><div class="ge-table-wrap ge-anim"><table class="ge-table"><thead><tr><th>Control Account</th><th>CAM</th><th>CPI</th><th>SPI</th><th>CV</th><th>Status</th></tr></thead><tbody><tr><td>CA-100</td><td>D. Brown</td><td>0.96</td><td>1.01</td><td>-£49K</td><td>GREEN</td></tr><tr><td>CA-200A</td><td>K. Lee</td><td>0.87</td><td>0.91</td><td>-£182K</td><td>RED</td></tr><tr><td>CA-200B</td><td>K. Lee</td><td>0.94</td><td>0.96</td><td>-£52K</td><td>GREEN</td></tr></tbody></table></div><br><p>The exception report flags CA-200A. Karen Lee has to explain why the electrical package is at CPI 0.87, turns out the subcontractor mobilised late and the re-sequencing added cost. She owns the variance narrative and the corrective action plan. No ambiguity about who's responsible.</p></div><h2 id="why-obs-matters-for-evm">Why OBS Matters for EVM</h2><p>Without an OBS, you have data but no accountability. You can calculate <a href="/en/earned-value/definitions/cost-performance-index">CPI</a> at programme level and know the project is over budget. But who's causing it? Which package? Which team? </p><p>The OBS answers those questions by creating a clean chain of accountability: </p><p><strong>Project Director</strong> sees the programme-level dashboard. CPI = 0.93. SPI = 0.97. Two red control accounts. </p><p><strong>Project Manager</strong> sees all control accounts. Knows which CAMs need support. Chairs the monthly review. </p><p><strong>CAM</strong> sees their own control accounts in granular detail. Knows which work packages are driving the variance. Writes the variance narrative and owns the corrective action. </p><p><strong>Site Supervisor</strong> provides the physical progress data. "We completed 340 of 400 linear metres of cable tray this period." That feeds up through the OBS into the EVM system. </p><p>Remove any level and the system breaks. I've worked on projects where the OBS only went two levels deep, project director and package managers, with no project manager layer in between. The PD was drowning in detail. The package managers had nobody filtering their escalations. Everything ended up in the PD's inbox, and decisions took twice as long. </p><h2 id="common-mistakes">Common Mistakes</h2><ol><li><strong>Building the OBS around the company org chart, not the project</strong>: The company hierarchy is irrelevant to the project OBS. A senior contracts manager who sits above the project QS in the company structure might have no role on this specific project. The OBS reflects project reality, not corporate politics.</li><li><strong>Changing the OBS mid-project without rebasing</strong>: If you move a control account from one CAM to another, the EVM history needs careful handling. The incoming CAM inherits the variance. You can't zero it out just because the person changed. If the transition creates confusion in the reporting, document the change formally in the EVM log.</li><li><strong>Too many levels</strong>: For a £35M project, 4 levels is plenty. I've seen an OBS with 7 levels on a £20M contract. By the time information reached the project director, it had been filtered through so many layers that the original signal was lost. Fewer levels. Faster information.</li><li><strong>Gaps in coverage</strong>: Every work package must sit under a control account, and every control account must have a CAM. If you've got £1.2M of preliminary costs that don't belong to any package manager, they still need a CAM. Otherwise, that spend is invisible to the EVM system. Somebody must own the prelims.</li><li><strong>OBS not aligned with subcontract packages</strong>: If your subcontract structure doesn't match your OBS, reporting gets messy. Ideally, each major subcontract maps to one CAM. When a single subcontract spans two control accounts (say, the cladding sub does both external envelope and internal partitions), one of those CAMs is getting incomplete cost data from the subcontractor's applications.</li></ol><div class="ge-product-note ge-anim"><p><strong>How Gather helps.</strong> Gather's AI reads your site diaries daily and maps progress against your cost-loaded programme, giving you accurate earned value data without manual spreadsheet updates. <a href="https://gatherinsights.com/contact">Book a demo</a> to see it working on a live NEC4 project.</p></div><h2 id="frequently-asked-questions">Frequently Asked Questions</h2><h3>What's the difference between OBS and WBS?</h3><p>The WBS breaks down the project by deliverables, what work needs to be done. The OBS breaks down the project by people, who is responsible for doing it. The WBS is product-oriented. The OBS is organisation-oriented. Where a WBS element intersects with an OBS element, you get a control account. Both structures are needed for a functioning EVM system. The WBS without an OBS tells you what's over budget but not who's responsible. The OBS without a WBS tells you who's on the project but not what they're accountable for. </p><h3>How detailed should the OBS be?</h3><p>Three to four levels for most construction projects. Level 1 is the project director. Level 2 is the project manager and commercial lead. Level 3 is the package managers or <a href="/en/earned-value/definitions/control-account-manager">CAMs</a>. Level 4 is site supervisors (optional, they provide data but rarely own control accounts). Going deeper than 4 levels on anything under £100M is usually overkill. The test: can each person at each level describe their EVM accountability in one sentence? If they can't, the OBS is too complicated. </p><h3>Does the OBS need client approval on NEC4?</h3><p>Not formally. NEC4 doesn't require the Contractor to submit an OBS. However, the Project Manager may request visibility of the project organisation as part of the communication and reporting requirements in Works Information. On some Network Rail and Highways England contracts, I've seen the Works Information explicitly require an OBS that maps to the WBS and identifies control account owners. Even without a contractual requirement, sharing the OBS with the PM builds confidence that you've got proper governance in place. </p><h3>Can one person be CAM for multiple control accounts?</h3><p>Yes. On a £35M project, you might have 8 control accounts and only 4 package managers. Each manager owns 2 control accounts. That's fine, provided they report on each control account separately. The danger is when one person owns so many control accounts that they can't give adequate attention to each. As a rule of thumb, a CAM shouldn't own more than 5 control accounts, and the combined <a href="/en/earned-value/definitions/budget-at-completion">BAC</a> of their accounts shouldn't exceed what one person can reasonably track month to month. </p></article></div>