Mutuality of obligation (MOO) is one of the three core elements of the employment status test. In broad terms it is the obligation on the engager to offer and pay for work, and on the worker to accept and personally perform it. It is part of the irreducible minimum from Ready Mixed Concrete v Minister of Pensions [1968] 2 QB 497, alongside control and personal service.
The modern position, confirmed by the Supreme Court in Professional Game Match Officials Ltd v HMRC [2024] UKSC 29, is that MOO can exist within a single engagement: once an appointment is accepted there are obligations on both sides. So the absence of any obligation to offer further work between assignments does not, by itself, put a contractor outside IR35.
The key point for contractors is that MOO is necessary but not sufficient. Even where MOO and control are both present, the tribunal must still stand back and ask, on the whole picture, whether the hypothetical contract is one of employment, including whether the worker is genuinely in business on their own account. HMRC's CEST tool treats MOO more narrowly than the case law, which is one reason a CEST result is not the last word.