I'm going to learn Clojurescript for building UIs and I want to use the React wrapper.Should I learn React before diving into Om?
Learn ClojureScript first, don't even think about React.
The learning curve for Om is not straightforward and you'll need to have a fairly grounded understanding of the fundamentals of the language before some of the abstractions that come with Om make sense (atoms, protocols etc). It's designed to be powerful, not simple and accessible.
Honestly, I'd try Re-agent first. The syntax for describing views is nice out-of-the-box and it'll also help solidify your understanding of atoms. If you like it, then you can complement it with Re-frame for a full blown FRP architecture.
I think it's easier to learn Om coming from Re-agent or Re-frame, than it is from React.