I haven't done OU I've only been to mainstream unis, and currently study distance learn for my masters from one, but I am going to assume that the marking system in general is different. I would assume (I've not heard of this before this thread though) that say for example you wrote an English lit paper identically and had it marked under the OU system and under a different uni that for example it would get 70% in the mainstream, but 85% in OU so although one is higher than the either it equates to the same thing, a first. I hope it is this otherwise it is definitely unfair!!!
I agree with wishyouwerehere in that distance learners probably get generally better marks, I was a 2.1 student through my undergrad full time, but now I study distance learning I am a "distinction" student. However, if distance learning generally creates better motivated students they shouldn't be penalised for it, they should be recognised as potentially better students if this is the trend and get the same marks as anyone else would, if you are going for a job your employer should know if you had the better quality marks regardless of how you studied.