EPA OOOOb vs OOOOc: What Is the Difference?
Operators keep hearing OOOOb and OOOOc used almost interchangeably, but they are two different instruments doing two different jobs, and confusing them leads to applying the wrong deadline to the wrong equipment. One is a federal standard that applies directly to new and modified sources; the other is a set of guidelines that only becomes enforceable once a state writes it into a plan. This page compares the two side by side, explains which of your facilities each one reaches, and shows why the practical monitoring work ends up looking similar even though the legal path is not.
OOOOb vs OOOOc in one line: OOOOb is the EPA new source performance standard that applies federally and directly to oil and gas sources built or modified after its applicability date, while OOOOc is a set of emission guidelines for existing sources that becomes enforceable only through a state (or federal) plan. In short: OOOOb regulates new equipment now, OOOOc regulates existing equipment through state plans on a later timeline.
The Core Distinction: New Sources vs Existing Sources
The cleanest way to keep OOOOb and OOOOc straight is to look at what triggers each one. OOOOb is a new source performance standard under the Clean Air Act, which means it applies to affected facilities that are constructed, modified, or reconstructed on or after its applicability date. If you build a new well site, install a new compressor, or modify an existing affected facility past that date, OOOOb reaches it directly as federal law, without any state action in between.
OOOOc takes the other half of the universe: sources that already existed before the new source date. The Clean Air Act does not let EPA regulate existing sources directly the way it does new ones; instead it issues emission guidelines and requires states to develop plans that implement standards at least as protective. So OOOOc is a floor that states build on, and the obligation an existing well site actually faces comes from the approved state plan, not from the federal guideline text itself.
This is why the same physical piece of equipment can fall under a completely different compliance path depending on one date. A compressor installed after the applicability date is an OOOOb affected facility governed federally; an otherwise identical compressor installed before it is an existing source whose obligations arrive later through OOOOc and a state plan. The equipment is the same; the legal machinery is not, and that machinery decides your deadlines.
Comparing the Two Rules Side by Side
The table below lines up the two rules on the attributes that actually change how you plan compliance. Both sit inside the same broader methane program and share much of the same monitoring vocabulary, which is exactly why they get confused.
| Attribute | OOOOb | OOOOc |
|---|---|---|
| Regulates | New, modified, reconstructed sources | Existing sources |
| Legal form | New source performance standard | Emission guidelines |
| How it binds you | Directly, as federal rule | Through an approved state or federal plan |
| Timeline | Applies from its applicability date | Later, once the state plan is in force |
| Who writes your obligation | EPA | Your state agency, using EPA guidelines as the floor |
Read the table top to bottom and the pattern is clear: the difference is not really about the technical controls, it is about who issues the obligation and when it lands. That distinction drives your compliance calendar more than any single monitoring requirement does.
For monitoring in the field the two converge. Both push operators toward finding and fixing leaks and toward reducing routine emissions from the same equipment categories - well sites, compressor stations, storage vessels, and the like. The requirement to survey for fugitive emissions and repair them, discussed in the guide on the leak detection and repair cycle, shows up on both paths even though the legal citation differs. That is why a monitoring program built for one is largely reusable for the other.
What Each Means for Your Monitoring Program
Because OOOOb binds new sources immediately, the practical trap is greenfield and modification projects: a new pad or a reconstruction can pull equipment under OOOOb the moment it comes online, so the compliance obligations need to be scoped during design, not discovered at startup. Fugitive survey schedules, control requirements on storage vessels, and monitoring for the affected facilities should be in the project plan before first gas, because the standard does not wait for a state plan to activate.
OOOOc is the slower-moving obligation for the fleet you already own, and the practical trap there is assuming existing equipment is exempt. It is not exempt; its obligations are deferred until the state plan takes effect, and when that happens a large population of existing sources can come into scope at roughly the same time. Operators who treated OOOOc as hypothetical then face a wave of survey and control obligations across an entire existing fleet with little lead time.
The monitoring work that satisfies both is continuous and data-driven rather than paper-driven. Whether an obligation arrives through OOOOb directly or OOOOc via a state plan, the operator still has to show leaks were found and repaired and that controls stayed in service. A platform such as Merobix supports that by trending the equipment parameters the rules care about and logging events over time, so the same monitoring records serve whichever legal path a given source sits on. For the legacy family these rules descend from, see the explainer on EPA Quad O and the OOOO subparts.
Frequently Asked Questions
Is OOOOc already in effect?
OOOOc is a set of emission guidelines, and guidelines do not bind operators by themselves. They take effect for a given source once that source's state develops and gets approval for a plan implementing standards at least as protective as the EPA guidelines, or once a federal plan applies. So the practical answer depends on your state's plan status. Until that plan is in force, existing sources are not yet subject to OOOOc obligations, but they should not be treated as permanently exempt because the plan process is underway.
Does OOOOb replace the older OOOO and OOOOa rules?
No. OOOOb applies going forward to sources constructed, modified, or reconstructed on or after its applicability date, while sources tied to the earlier applicability dates of OOOO and OOOOa remain governed by those earlier standards. An operator can therefore have equipment sitting under different subparts of the same 40 CFR Part 60 family depending on when each affected facility was built or modified, which is why tracking construction and modification dates per source matters so much.
How do I know which rule applies to a specific well site?
Start with the construction, modification, or reconstruction date of each affected facility on the site relative to the OOOOb applicability date. Sources on or after that date are OOOOb affected facilities governed federally; sources predating it are existing sources whose obligations arrive through OOOOc and your state plan. Because a single site can mix new and existing equipment, the determination is made per affected facility rather than for the site as a whole, so keep the dates and equipment inventory documented.
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