Can you use a spell like Searing Smite on an attack of opportunity if you haven’t hit with your weapon on your turn?

can you use a spell like searing smite or branding smite on an attack of opportunity if you haven’t hit with your weapon on your turn? to clarify: you have cast the spell already and are concentrating on it but have not been able to use it yet.

— FlutterNuffer (@Wafflereck) January 19, 2020

"The next time you hit a creature with a weapon attack before this spell ends…"

If the next time you hit a creature with a weapon attack before the spell ends is on an opportunity attack, they get smote.

— Dan Dillon 👥 (@Dan_Dillon_1) January 22, 2020