Any idea how to build around obstagoon?
Obstagoon needs partners, which can handle preferrably Fighting-types and Pokemon with Fighting-type Coverage, such as Kartana with Sacred Sword. It also needs alot of support to handle Volcarona and Tapu Fini, Tapu Lele, Clefable, Tapu Koko, even Tapu Bulu.
The best way to cover its weaknesses would be with Pokemon like bulky Dragonite as this can handle all the three mentioned Pokemon in Urshifu-R, Kartana and Volcarona pretty well. Specially defensive Landorus-T can also handle Volcarona and is able to set up Stealth Rocks and give momentum with U-turn and with access to Knock Off / Toxic it is a great partner. Dragapult can also be a great help with handling Urshifu-R.
Dragonite and your own Volcarona can also handle Buzzwole, the same mention goes to Tapu lele with offensively pressure it.
To the Fairy-types, the best way to cover them would be Steel-types such as Corviknight, it can check Tapu Lele, Tapu Bulu, and Clefable decently well. Heatran can also check Clefable and Tapu Lele and furthermore can check Tapu Koko in a pinch.
Other great sources of counterplay would be Galarian Slowking, as it handles Tapu Fini, can check Tapu Lele and Tapu Koko as well as Clefable. It can also help Obstagoon in making progress due to Future Sight, and it can lure in Landorus-T, Garchomp, and Dragonite with Ice Beam, which is -- especially in Landorus-Ts case -- valuable to open a path for Obstagoon.
Other great teammates would consist of Pokemon such as Mew for the potential to stack Spikes with Taunt, Knock Off, and Night Shade, it can also check Tapu Lele in that aspect and Hazard Stack helps Obstagoon opening up a path due to the pressure Hazard Stack puts on the opposition.
Scizor, Kyurem, and Tornadus-T can also be great partners for Obstagoon to offensively pressure the opposing teams.
These are the Pokemon to cover and a way to cover these threats. There are more great Pokemon which can help Obstagoon such as Rotom-W, and your own Kartana, Tapu Lele or Tapu Koko. As they all help pressuring the opposition very well alongside Obstagoon.
Hope that helped :)