I was under the impression that there were very few courses available from the OU to North American residents. However, on the page below, when Canada or the US is selected in the drop down, there are actually a fair number of availabe courses displayed, especially at the graduate level. For example, it appears as if all of the courses required for the MSc in Mathematics can be taken by Canadian and US residents. http://www8.open.ac.uk/study/explained/where-can-i-study I don't know if the OU recently made some changes to where their courses are offered, or if I just missed this before.
It's possible that the number of courses is expanding but degrees continue to be inaccessible for US citizens.
I have been accepted to the MSc Mathematics program at the Open University. Currently, I have completed only one course (3 credits) towards the MS in Industrial Engineering at NMSU. I did enjoy this course (Multivariate Statistical Analysis), but for me the goal of earning a master's degree is more for personal achievement rather than career advancement. I am more interested in pure math than stats or operations research, so I am considering switching to the OU program.