Mutuality of obligation (MOO)

Mutuality of obligation (MOO) is one of the irreducible-minimum tests for employment status. It asks whether the engager is obliged to offer work and the worker obliged to accept it — beyond the current piece of work.

Genuine outside-IR35 contracts are typically discrete: a defined deliverable, no obligation on either side once it's done. MOO points toward inside IR35.

HMRC has long argued that some level of MOO exists in any contract. Tribunals have pushed back: ongoing obligation beyond the current task is what matters, not the bare fact of a contract.

Related terms

• IR35 — The UK's off-payroll working rules — they decide whether a contractor should be taxed as a deemed employee.
• Inside IR35 — An engagement that falls within IR35 — the contractor is taxed as a deemed employee.
• Right of substitution — The right to send a qualified replacement to do the work — strong evidence of being outside IR35.
• Supervision, Direction and Control (SDC) — The client's right to direct how, when, and where you work — high SDC is strong evidence of inside IR35.