Can I apply for an O visa as a dependent while my partner applies for an OA visa in London?

sorry to say, we have no reports yet, of successful applications to a "trailing spouse" Non-Imm-O Visa for same gender married couples. You should ask your question accuratley and politely formulated, by an email to the Royal Thai Embassy in London. This is the only way you can get 100% reliable information onto how you can obtain a longstay permit in Thailand and your partner possibly be able to go "piggyback" on your 1-year Extension of Stay . . This group cannot give you any hints, as the legalization of same sex marriages just happened a few months ago, and the relevant authorities, mainly the Immigration offices in Thailand, might not yet be ready to cope with such new rules