I am thoroughly unimpressed with OneD&D, feeling that most of it is a combination of lateral moves and downgrades from 5e designwise, and what actual improvements there are tend to be blindingly obvious hacks. Nevertheless I really don't see how it is incompatible really at all, at least at the moment. You (at least generally) can't mix a 5e subclass with a 5.5 class, and some basic common sense has to be used in terms of mixing races and backgrounds of the two systems.
But the core rule changes seem so superficial I really don't see any reason yet why someone bringing a character with a OneD&D class to a 5e table would be a problem, or vice versa. They would still need a copy of the core rules from the system the table is using handy, so it's a bit inconvenient, but given that the current system practically begs players to truck around a PHB, Xanathars, Tashas, and whatever book they dug their race out of, bringing two PHBs doesn't seem like a problem.
My only real problem with the arrangement is that if the player running a 5.5 character is someone who needs help or monitoring I won't be able to provide it without learning this shoddy remix of 5e. I've spent my 5e days as the helpful rules reminder and explainer guy, but don't really care to learn a second variation of every class mechanic just so I can continue that after WotC's pointless brand refresh. It's going to be a tough transition.
