Can the space shuttle use its OMS engines if the shuttle is short during the runway approach? Is fuel available for use in such a situation and will the engines provide reasonable thrust to get to the runway?
As mentioned by @GremlinWrangler in a comment, the Orbital Maneuvering System (OMS) engines could not be fired below 70,000 ft (21 km) altitude.
The minimum altitude for an OMS engine burn is 70,000 feet. Below this altitude, the pressure difference between the inside and the outside of the OMS engine nozzle could cause it to collapse.
Source: Orbital Maneuvering System training manual, p. 3-18