Jamie the landlord is very friendly, and is almost always open to musicians going in and playing, as long as there's nothing else going on. He also runs the James Street Tavern. The Half Moon hosts legendary sessions on Sunday nights, 8pm til late. It's mixed tune and song, sometimes leans more towards Irish, sometimes English, but anything goes.
Address: The Half Moon, 18 St Clements Street, OX4 1AB
Contact: http://halfmoonoxford.com/hm/index.html 01865 247808 (pub)