It depends on the course, who's presenting it, and where it's being presented. The OU's own FutureLearn is probably best to avoid if you're interested in gearing up for proper study. They're strictly taster sessions, and not built at all like evaluated distance courses. edX MOOCs can be better, and I've heard of some good Coursera ones, but haven't taken any there personally.
I'd give serious consideration to looking at MIT's open courseware modules. They're not perfect, as they're developed around the lecture-and-contact-hours model and presented online, but the coursework itself can be exactly what you'd expect at university level. Other universities sometimes put their courses online, as well, but I'd look for ones which say they're the actual modules taught at the university. This can include requiring external texts, however.