Would it be odd to refer to English-style tea as お茶?

Question

If I were drinking English-style black tea, made by first placing a teabag in the cup, adding sugar, pouring boiling water on to it, stirring, adding milk, and stirring again, and a friend asked me, 「何飲んでる？」, would it be odd to reply, 「お茶だよ」? Would another term like ミルクティー be more appropriate?

Answer 1

Black tea is usually referred to as [紅茶]{こうちゃ}.

a friend asked me, 「何飲んでる？」, would it be odd to reply, 「お茶だよ」?

お茶 might be understood as Japanese tea (like, 麦茶{むぎちゃ} or 煎茶{せんちゃ}).

Would another term like ミルクティー be more appropriate?

Yes, ミルクティー is also common. So I'd recommend using 紅茶 or ミルクティー to avoid any confusion.　

(That said, you'd still say 「お茶しない？」「一緒{いっしょ}にお茶でもどう？」 etc. to invite someone out for tea or coffee... here you don't mean you're gonna drink Japanese tea at a cafe.)