In my experience, it has depended on the complexity of the integration and the preference of the business sponsors of the deal. Typically, the executive sponsor of the deal (usually at the SVP level) is committee ‘chair’, and then there is representation from the major work streams relating to the integration (finance, HR, IT, sales, etc.). The IMO is responsible for preparing the materials for and facilitating the meeting so much so that the steering committee meeting would not happen if it wasn’t for the IMO driving that interaction with the leaders and the discussion, so in a sense, the IMO is the shadow leader of the steering committee.
Those who have authority to make a call in key business decisions related to financial / operational / human resource should be part of the SteerCo.
IMO and Functional Leads (SMEs) can support the decision-making of SteerCo members.
It depends on how the organization is structured and the size and complexity of the integration. If the organization is global with multiple divisions and legal structures the steering committee may be made up of the CEO and CFO of the country and/or legal entity involved in the integration plus a selection of their corporate/function leads (ie HR, Legal, Division Head, etc…). I agree with the above comments that those on the steering committee should possess the authority for decision-making.