German carrier Air Berlin's eventual membership of Oneworld would bring the number of partners in the alliance to 14.
The group has the same total number of members, and members-elect, as SkyTeam but still only half that of Star Alliance.
Air Berlin and Oneworld are planning to disclose further details of their arrangement tomorrow.
Oneworld says simply that the German carrier has "accepted an invitation" to join.
But the airline would substantially reinforce the alliance's presence in Central Europe, where its reach is limited to Hungary's Malev.
It would also give Oneworld a stronghold at the new Berlin Brandenburg Airport which is scheduled to open in mid-2012.
Air Berlin, which has around 150 aircraft, owns the Dusseldorf-based long-haul operation LTU and is also a significant shareholder in Austrian carrier Niki, located in Vienna.
Its route network primarily covers Europe but extends also to several destinations in the USA, parts of Asia, as well as northern Africa and the Middle East.
Air Berlin is already adding to these points through newly-agreed codeshares with Oneworld members American Airlines and Finnair, which will come into effect on 1 November.
Oneworld's overall membership currently extends to 11 full members, with Russia's S7 Airlines and India's Kingfisher Airlines among incoming candidates.