Can I move my wallet.dat from Ubuntu to Mac OS? I want to run Bitcoin Core on Mac OS. 
Can I move my existing wallet.dat file from Ubuntu and use it in Mac OS?
I am using Bitcoin Core.
If you are using release binaries, yes. You must make sure to shut down Bitcoin Core cleanly before moving the file over, though.

If you're using self-compiled versions, this is only possible if both versions are compiled against the same BDB library.