I transferred some credits in to an Open Degree and it was very straightforward (although I believe the rules have slightly changed since I did so, though not in any very significant ways). Be aware that the grades themselves won't count towards the final classification of your Open Degree, though; this would be based solely on your OU study.
And yes, in theory you can study whatever subjects you like towards the degree, but they are starting to introduce prerequisite modules for some subjects so this could have an impact. Basically, they don't want people to suddenly decide to study something at Level 3 that they know nothing about and then fail, which is fair enough. So, the openness is moderated a bit by the progression of your knowledge. How far are you through your current degree, and what subject(s) do you want to do with the OU? My guess is that you can pick up some (but not all) subjects from Level 2 without too much problem, but it would be hard to jump in at Level 3. But check with the OU, they can advise you properly.