Apparently Dondozo + Tatsugiri is great in Doubles... but I don't know enough about those formats, so here's a generic wall moveset instead.
Gen 9 OU
Dondozo @ Leftovers
Ability: Unaware
Tera Type: Grass
EVs: 252 HP / 252 Def / 4 SpD
Impish Nature
- Rest
- Sleep Talk
- Body Press
- Yawn / Wave Crash
There are three Unaware Pokemon in OU currently (Clodsire and Skeledirge are the other two), so Dondozo has competition, but it can be justified on a team over the others if you really need a bulky Water type. It has the best physical bulk of the three, so it helps invalidate Baxcalibur and most other non-super effective moves. Dondozo also acts as a decent Chi-Yu check, but its passivity makes it easy for Nasty Plot variants to set up on it.
This is a pretty standard RestTalk set. The EVs give it crazy high bulk (504 HP and 361 Defense!), so it's fairly straightforward to heal against a physical attacker if need be, especially with Leftovers. Body Press means no EVs need to be invested into Attack, and it's great against Chi-Yu, Chien-Pao, and other foes weak to Fighting. Yawn helps with controlling momentum and forcing out setup Pokemon like Dragon Dance Dragonite. You could also use Wave Crash instead so you aren't completely walled by Annihilape and Gholdengo, plus the damage can be healed off with Rest. Tera Grass lets Dondozo soak up Electric and Grass moves while still resisting Water.
Good teammates
Chi-Yu
Chi-Yu is great against Grass and Ghost types that Dondozo can't do much against. In return, Dondozo can deal with Tyranitar and other Pokemon that might stop a Chi-Yu sweep.
Gholdengo
In the same vein as Chi-Yu, Gholdengo is solid against Ghost types. Dondozo also appreciates it soaking up statuses if it's not in a position to Rest.
Clodsire
Why stop at one Unaware user? Clodsire is a good switch-in to Iron Hands due to the Electric immunity, and its special bulk complements Dondozo's physical longevity. It should be noted that Gen 9 OU is a very offensive metagame, though, so more offensive options might be more viable.