Don't forget that the OAT integrated course is ab initio and so most people on it won't actually be able to fly at all. It is desgined to take you from no hours to CPL standard. As such I don't think you will have much problem if you don't fly before it.
As for doing the ATPL groundschool, if you do them yourself before going to OAT then you won't be allowed to do the integrated course, you would have to go down the modular route.