I don't see why it would be a bad thing as long as you're confident you could manage the work load. It will look impressive on your Uni applications, but if you somehow didn't manage to reach predicted grades etc and they see that on your application, they will undoubtedly assume it was because of the extra studying, so in that way it would be wiser to just concentrate on your a levels and get the best grades you can.
But if you really want to do it, then go for it. It's great that you're making an extra effort and Unis will see that.
I'm not sure if you will be able to get financial support seeing as you're a full time student, but I'm not 100% sure on that one. I think you also have to be over 18 to study with the Open University.
BUT I also have a feeling that if your parents were willing to pay the money for you, or you were able to pay somehow, and you talked it all through maturely with your parents and then got in contact with OU, they might consider you doing the course in special circumstances.