Almost all OU modules now start in October, with the exception of a few Level 1 modules that have more frequent starts, and the odd February start module in some subjects (although the trend seems to be that these are being phased to October starts, too). And, even if you have an October start and a February start, this means there's a fair bit of overlap (February - June) where you're doing both at once.
So, yeah, basically if you want to do full-time you will inevitably have to do more than one module at a time.