Following MECO, the orbiter's altitude and velocity will vary depending on the mission requirements. For example, an 80-nautical-mile (92-statute-mile) altitude with an inertial velocity of approximately 25,660 feet per second would place the orbiter in a suborbital trajectory so that the ET would enter following separation. In order to boost the orbiter to a viable orbit that does not degrade appreciably during the mission and satisfies mission objectives, two propulsive thrusting periods are made with the OMS engines, except in the case of a direct insertion, when only one OMS thrusting period is required to circularize the orbit. The first thrusting period is referred to as OMS-1 and boosts the orbiter to the desired apogee; the second burn is called OMS-2 and typically circularizes the orbit.

The optimal orbital altitude (the altitude that satisfies mission and payload goals) is determined before launch. During flight, however, problems, such as main engine and SRB performance loss and OMS propellant leaks or certain electrical power system failures, may prevent the vehicle from achieving the optimal orbit. In these cases, the OMS-1 and OMS-2 burns would be changed to compensate for the failure by selecting a delayed OMS-1, AOA or ATO abort option.

The main events that occur during the orbit insertion phase include execution of the OMS-1 thrusting period, typically about two minutes after MECO; an MPS propellant dump, which begins during OMS-1; positioning of the main engine nozzles for entry; shutdown of the three auxiliary power units; MPS power-down; and MPS vacuum inerting to ensure that all traces of MPS propellants are vented to space.