Jim,
The OTR system is sealed. You cannot add oil. It is installed prior to charging the system. If it leaks out you are leaking out your freon charge. Typically if the AC compressor has an oil leak it will show as a dirty oil stain behind the clutch coming from the shaft seal. It will not be wet, as much as it will just be a dirty film of oil and a small amount is likely to be found on every bus with OTR.
If your OTR blows cold, rest easy. You do not have a problem.