Oxygen levels wouldn't be a factor. The "need high oxygen levels for large size" only applies to invertebrates with very simple breathing systems.
If trex had similar type of lungs as modern birds, they would do just fine. Bird lungs are super efficient (much better than mammals' puny blind sack lungs) that enable them to fly at high altitudes.
Playing FTL taught me that when you need to reroute power, life support is the first to go.
Keeping weapons hot, engines full and shields up make you dish out or avoid damage. The crew can hold their breath.