oneworld is losing out in Eastern Europe to Star & Skyteam. Star already has LOT (and Adria & Jat as associated members) and Skyteam has CSA. BA doesn't have a huge market penetration in Eastern Europe, although I'm not sure about Finnair. Malev will obviously have to improve their services if they want to join oneworld, but I'm sure they will bring a much needed expansion to oneworld.
Malev have been a long-time partner of British Airways on flights between the United Kingdom & Hungary, and I'm sure that the same stands for American on flights to the USA. Also, aren't Malev's longhaul routes already into AA city hubs?