Yes, you can access, and therefore use, BA's lounge at Changi when travelling on another oneworld carrier. Do you know where it's located (Level 3 of the Departures Lounge of T1)? It can be tricky to find. And beware that you will have a long, long way to travel to get from the lounge to your Qatar gates. (use the Skytrain). Be careful you don't miss your flight.

http://www.ausbt.com.au/ba-opens-new...business-class
http://www.changiairport.com/imapswf/imap.html