Referencing back to the compatibility issue, I have a way to look at this that may help some people.
When the "One D&D isn't compatible" discussion on this thread started, someone mentioned Dragonborn and Goliath not being compatible, because they have new abilities. However, this leads me to a question.
Was Fizban's Dragonborn compatible with 5e DnD?
Because before Fizban's the Dragonborn had resistance and an action breath weapon, and basically nothing else. In Fizban's the dragonborn had resistance, five new potential energy types, attack breath weapons that scaled diffrently, and new abilities including immunity, a massive AOE shove, AND LIMITED FLIGHT. The Gem dragonborn flight is basically identical to the version in One DnD.
So, if OD&D is not compatible, because Dragonborn have flight as a new ability... wouldn't that have meant that Fizban's wasn't compatible with DnD 5e either?
Now, a stronger argument can be made for the cleric. They did rearrange some of the abilities... but again, not really incompatible, is it?
Let us say that I wanted to play a One D&D cleric with the Grave Domain. We haven't seen this one playtested, but it would be rather simple for me to do. At 3rd level I'd get Circle of Mortality and Eyes of the Grave features. The Domain spells would cut off the 1st level spells, but otherwise I'd get the same spells at the same levels.
6th level, the Channel Divinity: Path to the Grave feature. 10th Sentinel at Death's Door. 14th, Keeper of Souls.
Yes, there are changes, but as we keep saying "compatible =/= identical"
The only class of the four we have seen so far, that I could not trivially work into the One D&D framework, is the Bard. Because, to date, it is the only class that only has THREE subclass features in normal DnD, where the One D&D standard is four.
Now, this is a compatibility issue, but one that can be solved by simply having a default bard feature to plug into the level 10 slot. Something any bard can have to replace their normal feature with.
Looking ahead, the only other classes that have subclass issues like this are the Fighter (five subclass features) and the warlock because of their dual-subclass nature and unusual class design. Neither of which we have actually seen yet, so we can only speculate on whether or not it will be an issue.
I will admit, The Fighter is a tricky one. My best current solution would be to combine two of the subclass features, and give them both at the level. But, since we haven't even SEEN the fighter class yet, we don't even know if that will be necessary
And that's... really it. Nothing else we have seen is "incompatible" with 5e. Feat at level 1? Already exist in 5e. Most of the "changes" exist to one degree or another, or are things like changing the rules for grappling, or how a spell works. Which are perfectly compatible, because they are no different than a Grave Cleric saying "now you can cast spare the dying as a bonus action with a range of 60 ft". It is a change, but it is compatible with the game.