That skarmory set is more used in Ubers, where it can be used to escape Mega Gengar. Replace item with Leftovers or Rocky helmet. And counter with brave bird
Galvantula doesn't really belong on a serious OU team.
I recommend taking a look at this: https://www.smogon.com/forums/threads/usum-ou-ultra-viability-ranking-thread.3621329/
The Garchomp set does too many things at once. Either have tank chomp or swords dance rockium z chomp
Hoopa-U isn't something you slap on a team. It requires a lot of support to be good in OU. You should include some form of volt-turn support. I highly recommend looking through the smogon forums and look at some good OU team samples. https://www.smogon.com/forums/threads/usm-ou-sample-teams-v2-up.3638845/
Also here's a general guideline of threats to watch out in OU:
Greninja (Ash and Protean)
Volcarona, Charizard Y, Greninja are threats
Your team doesn't have to reliably check all of these threats, but it should cover most of them
The smogon dex also has some good functionality: https://www.smogon.com/dex/sm/pokemon/
Under each pokemon for a certain tier, you're able to find a list of team partners that could be helpful.
Here are some other resources that might be helpful
There are some things that you just get a feel of, like if you have Toxapex, then Tapu Bulu, and Tornadus-T are typically followed because it's such a natural construction
This is assuming if you know the basics, if you don't, I recommend sumwun's guide: https://docs.google.com/document/d/1qYztMbBTY0qm4mFy1-66bafgsVQ2gkeErAO3QUghEBY/edit?usp=sharing