Green’s Dictionary of Slang

extravagant, beyond the usual bounds of taste, behaviour etc.

1982		Barr & York Sloane Ranger Hbk 159: OTT adj. Over the top—outrageous. Usually ‘absolutely’ or ‘totally OTT’.
1999		Guardian G2 2 Aug. 6: Posh’s wedding was ‘OTT’.
2000		T. Udo Vatican Bloodbath 97: Nuking a city of 10 million souls into radioactive slag? Isn’t that a bit, well, y’know, OTT?
2005		P. Howard Curious Incident of the Dog in the Nightdress 184: She gives me, like, this huge hug, which is a bit OTT.
2017		theculturetrip.com ‘Guide to London Slang 10 Jan. 🌐 Extra – OTT basically.
2024		J. Meades Empty Wigs (t/s) 848: That’s where his anti-Semitism comes from. A tad OTT as reactions go I’d say.