original poster; OP; thread starter
Internet slang for the person who started a forum thread or discussion topic. Common on 2channel and similar boards.
See also: スレ
The OP posted the first question.
スレ主の意見に賛成する人は多い。
Many people agree with the thread starter's opinion.
Compound of スレ (from English 'thread') and 主 (ぬし, 'master' or 'owner'). The reading ぬし for 主 is a native Japanese word.