First off, these are 2 great themes for different reasons. For quality code and experience, GeneratePress wins out. Tom, the GeneratePress developer, is a very talented and experienced developer who is passionate about every line of code in his theme. I have never experienced anyone more committed to making a quality theme than him. OceanWP is a more feature rich theme, more options, but more complexity to it. If you were only going to use the free versions of either, OceanWP is what you should choose. Its strength is WooCommerce integration. Both themes have very good and fast support. If you were going to be purchasing one of them, GeneratePress offers more value at a much better price tag. So I can really pick a winner here, but can suggest you make your decision based on what your plan is.