I think おっさん臭い literally means 'old man stench', if I'm not mistaken, but it seems like the actual meaning is different.
For example, in this episode of ちびまる子ちゃん Tomo says
"なんかちょっとおっさん臭い気がするけど"
after (if my listening comprehension is correct) a classmate with a weirdly formal style of speech tells her that doing certain eye exercises will open up a 3rd eye that can predict the future.
https://www.youtube.com/watch?v=MNi54uQEr60&t=366s
And then in the show 推しの子 at the end of episode 1 at timestamp 1:19:13(spoilers for episode 1)
Aqua, the brother says:
> "スカート短すぎないか?"
To which his sister Ruby replies :
> "お兄ちゃんって昔からおっさん臭いよね"
Is おっさん臭い used to mean 'creepy', 'weirdo', or does it have some other meaning(s)