If I said I have very little understanding of the Gen 7 pokemon, would this change your answer?
It probably shouldn't in regards to Gen 7 OU vs a different tier - pretty much every tier has some of the Gen 7 Pokemon as relevant forces, whether that be PU with Oricorio and Type: Null, NU with Incineroar and Passimian, RU with Salazzle, Golisopod, and Zygarde 10%, UU with Primarina and Muk-Alola, or OU with the Tapus, Magearna, and Kartana.
If you have an interest in learning Gen 7, then I'd advise looking into the different tiers to see what appeals most to you. Look at their Viability rankings to get a general overview of the meta and look at replays to see what the games often look like. OU certainly has one of the largest playerbases, but you should go for whatever tier you feel is most interesting.
If you're not particularly interested in Gen 7, then there's definitely a fairly active oldgens community out there - if you're looking to get into pastgens then you should check out the
Ruins of Alph subforum which has a bunch of resources and visit their PS room and discord. If you're looking for a different pastgen tier than OU like Gen 4 UU then you might need to look harder to find help getting into the metagame, but you can probably find people willing to help you in that tier's PS room or discord.
Either way, pick whatever you feel is most interesting to you and float around if you're unsure, you can't really pick a "wrong" tier to start learning!