Both Qatar Airways and SriLankan Airlines are expected to join the Oneworld alliance sometime in the late 2013/early 2014. The joining dates for both of the airlines have not been confirmed yet.
Now, you can, however, start using these two airlines on Oneworld alliance’s Global Explorer fares. At the same time, the gulf carrier, Gulf Air has withdrawn its participation on these fares. Gulf Air has never been member elect or future member of the alliance. You can read the press release about this announcement here.