I'd get a case anyway, what is this jelly issue?
Oh, I heard about this issue. Apparently it was a thing where if you scrolled something on the screen vertically very quickly, you'd see a sort of jelly effect where the objects on the screen would visibly grow and contract in response to the scrolling, like you were jiggling a big piece of Jell-O or something. I didn't have the issue to any noticeable degree (apparently it was a thing that happened after the very first batch of phones went out), so I can't tell you how bad the issue is or whether OnePlus will do anything about it if you are afflicted.
I think if you get a case with the OP5, then you're pretty much golden. The biggest issues I can think of at that point are 1) Dash Charging is proprietary, so Quick Charge power banks, chargers, etc. don't work with it, and 2) I guess the GPS maybe drifts a little sometimes on Pokemon Go, which makes raiding annoying. These are not really reasons to not buy the phone, in my opinion. I'd definitely get it over an S8 for sure.