I had no idea that "Oh my days" was some sort of wannabe gangster speak.
I'm in my late 60s and have used it in situations where "For fuck's sake" or "Fuck me sideways" wouldn't be appropriate for as long as I can remember. I lived in Croydon/Streatham/South Norwood for the first half of my life though, so wonder if it might be a south London thing as much as an urban mulitcultural thing. I can remember my late mother saying it, and my family aren't multicultural at all (well, unless having 2 Irish grandparents counts).
I hope people don't think I'm trying to be cool and down with the kids or anything when I say it.
I'm with PPs on "picky tea" though, and not keen on "poorly" either. I also dislike the word "loo" quite intensely. It's a mimsy, childish sounding word imo, and it makes me wince inwardly. I don't even like the way it looks on the screen.