Oliver was far from stuffy, and if Charles wasn't going to correct him, or banish him, or whatever fancy British people did, he assumed it was cool just to treat him like he treated everyone - like a normal dude. A pal. Besides, he thought Charles seemed pretty cool. He was the only one who didn't seem to disapprove when he'd shown up to dinner drunk on the night of the murder.
"Cool, thanks!" Oliver said, all formality quickly forgotten. "Are you gonna join me in here? The water is great."