Typically "O" rings are a seal and used so there are no leaks. I would be sure to find out what he removed and if they were definitely "O" rings, have him put them back.

@millworkman thank you, that’s what I was thinking. So they are there to prevent leaks? Do you think he could have meant that he took off a flow restrictor or something? I’m just so surprised that a professional plumber would remove something so necessary.

" Do you think he could have meant that he took off a flow restrictor or something? "

No idea.

Ask him what he actually removed. I suppose he could have removed them when he cleaned out the aerators but are you sure he didn't actually put them back on afterwards?

The O rings just seal the screw-on part. If it's not leaking, you aren't missing (or don't need) them. It's an obvious leak that won't sneak up on you.

He probably removed the flow restrictors. I did this on my hand held shower.