Harves,
Although I've got a month before I start my course at OATS, I'm under the impression that the initial ground school part of the course is pretty hectic. I'm not convinced there'd be enough time to do any hour building alongside ground school - the time might be better spent working towards first time passes in the ATPL exams...
I'm in a similar position in that I've been starting on my PPL and I'm gutted that I'll have to study for 19 weeks before I can start flying. Then again, I'll have 3 months of flying for the next stage of the course before returning to ground school.
At the end of the day, both courses get you to the required number of hours....