I emailed OU asking the same question yesterday. Their response was although maintenance loans are available to part time students it is only to those who attend brick universities, not the OU. I have also read elsewhere that it is only open to new students, not those already part way through their course. I'm not sure how accurate that information is though.
However, it looks like OU students may be offered the loan in 2019/20 but like I stated above, I'm not sure if that will be available to just new students or those already studying for their degree's. If it is just open to new students I'm sure some will want to delay enrolling on to their courses until next year and I would be one of them. Not great as the whole idea of offering the loans to part time students was to stem the drop in take-up of part time courses.
The SFE site doesn't even mention the fact that OU students aren't eligible so don't expect much help from them.