Most modules run from Sept/Oct to June, or Jan/Feb to Oct. Some have start dates for both periods, some will only run for one of those timings. So depending on which modules you choose, the OU can be a year-round thing. I'm in my fourth and final year of my degree and only had one gap as far as I can remember, which was from Oct 2012 to Feb 2013, when I started my last 2 modules. Other than that my modules have been running continuously.
(some modules also start in April or June I believe but I don't think there are many with that start date)