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.
If it’s T-Rex in present day no thank you. You would die a painful quick death due to not having enough air to breath even at sea level.
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.
So what you’re saying is that life…uh… finds a way.