I think (and I may be wrong here) you can study as many credits as you like. However, every level of an honours degree is 120 credits. So 120 credits is "full time" study. So I really wouldn't recommend studying more than that (and it may be the case they don't allow it anyway). Even studying full time, with a full time job if you have one, will be a huge struggle.
But you can certainly study up to 120. You may be able to take level two and one at the same time, but will need to make sure you have prepared adequately for the level two course (any relevant level one modules that may be a help). Sorry, I've explained that really craply. I'm a bit tired at the moment so will try and rephrase it later.
Have you booked and appointment to see an OU adviser? There may also be drop in sessions in your local area. They are the best people to ask.