Mutuality of Obligation is a subject wide open to interpretation and misunderstanding.
Of course, in all commercial relationships and transactions, there must be some mutuality of obligation. The whole point of having a contract or agreement is that the respective parties are obligated to each other, within limits.
For IR35 purposes, the problem arises when the obligations between the worker and the client are badly defined or are extended beyond the limits of the original agreement. HM Revenue & Customs will view long-term engagements with suspicion, because in such situations the relationship between the worker and the client might develop to such an extent that the client expects the worker to accept any work offered, and the worker expects a continuous supply of work, which is characteristic of employment.
To avoid a MOO problem, you need to ensure that the services provided to the client are precisely defined and recorded, so that it is clear that no work is to be done beyond that agreed formally in advance. It is helpful to have definite start and end dates. If the contract is extended, this should be properly documented, with the new services clearly defined. You should avoid any suggestion that you will provide a general service or undertake work beyond the precise limits set by the contract.