Never Say “Oops”
“Oops” is so frequently abused as error message copy that you would lose nothing by banning it entirely from your product.
“Oops! You need to log in first.”
I’m sorry, but where was the whoopsy? No one messed up. This is normal.
“Oops! That page doesn’t exist.”
The page was intentionally deleted. The user intentionally clicked a link that was once valid. Where’s the oops?
It’s much better to be factual. Apologize if you want to: “We’re sorry, you’ll have to log in first.” Or, “That page doesn’t exist or was deleted.”
Update, April 18, 2018: The time when “oops” is perhaps most appropriate is when the user has guessed a URL that does not exist. Still, rather than mocking the user, we should give them a helpful 404 page. (I have set up alanhogan.com to do this, for example; I suggest a site search with terms extracted from the non-existent URL.) For more thoughts about being nice to users who try to navigate your site by URL, please read URL as UI.