Riffue Premium Thin Shell
If you're afraid of using a case because they're all too bulky, consider the premium thin PC hard shell case from Riffue. This is one of the thinnest cases on the market for any phone, including the OP5T.
Riffue uses an extremely thin polycarbonate plastic that's precision cut to match the OnePlus 5T. It's so thin that the company states that buyers shouldn't bend or flex the case during the installation. Carefully snap the phone into the case, and you're all set.
However, unlike most thin polycarbonate shells, the company adds a rigid texture to the back for increased grip. That way the phone isn't too slippery. Keep in mind that this type of case doesn't offer a ton of "drop" protection, but will keep your phone safe from light dings and daily wear and tear. If you're worried about dropping it, choose something more durable.