The officious bystander test is used to determine whether a particular term, that was not expressly agreed to, was nevertheless intended to become part of the contract by implication.
The test is hypothetical and objective. The court imagines that, when the contract is being made, an 'officious bystander' asked the contracting parties whether the suggested term is part of their agreement'. The court then infers, from the objectively known facts, whether the parties would have answered 'of course', in which case the suggested term is considered sufficiently obvious to be implied ad hoc.