Most institutions will agree that an Open University Cert.HE is not on par with those from conventional universities. This is why it is rare to accept a first year OU student into a second year programme somewhere else.
A lot of incorrect assumptions are being made. One has been that if the final degree of two three-year institutions are equivalent, then so must each of the three years. (This is not the case as the starting points are different.) One has been that UCAS and OU course codes were the same thing. One has been that an OU degree takes four years. One was that all universities break down mathematical concepts in the same manner.
Now there's an assumption that education at conventional universities occurs at a linear rate for each of the three years. (Ask any third year student how much they miss their first year.) There's another that the first year of the OU brings students up to A-level range. (There's already a distance learning alternative to A-levels called the National Extension College, it was set up as a pilot programme for the OU, and it's much cheaper. The OU starts at the same place as the NEC, and goes much quicker.)
To answer your question (which makes more assumptions) about the information in B1, some of it is learned by OU students at Stage 1, and some at Stage 2. The information in B2 is learned some in Stage 2, and some in Stage 3. The information in B3 is learned in Stage 3. All three years cover a higher percentage of the degree than at a conventional university. Imagine that the degree is 100%. Pretending that the learning is linear, which it's not, OU students would learn an average 33% of their degree each year. Conventional students would have some portion, which we'll arbitrarily pretend is 25% just because having any number to illustrate makes it easier to understand, and because it makes the maths easy, already under their belt due to A-levels. They'd then cover only an average of 25% of their degree each year.
Please don't draw the numbers out to make more of a point of them. They're arbitrary and wrong, and only there to help wrap minds around it. A first year OU student will gain some knowledge beyond A-level, however, and be far behind starting the second year at a conventional university. A second year student will gain some knowledge beyond a first year student at a conventional university, but be a little behind starting the third year at a conventional university. By the end of the third year, the information they have accumulated in either recognised institution has been evaluated by accreditation bodies to have conferred the appropriate amount of knowledge for a degree.
It's your choice whether you trust the accreditation bodies or not, but that's all it really comes down to.