The way to succeed doesn't depend a lot on which university you attend, nor does the field you want to study, though the specifics differ a lot. The key to it is to understand how the human brain works.
First, every learner is different and some things are easier and some are harder for different people. Hardly anyone, however, will actually learn anything by seeing or hearing or reading it just once. The brain works through repetition and requires actually physical changes. New synapses must be established between neurons and that takes reinforcement to have any chance of retaining new information and skills in long term memory. See The Art of Changing the Brain by James E Zull for the scientific explanation.
So, the key, in a field like language or history is to look at a lot of issues and write about them. It is the writing that provides the reinforcement, not the "looking". Write a lot of short papers, poems, speculations, whatever. In mathematics, you solve a lot of similar problems. In CS you write a lot of programs. But the learning has to be active - producing something - not just passively consuming it.
But, the brain can be misled. You can reinforce the wrong thing. You also need to avoid that. So, in addition to reinforcement to learn, you also need feedback. Someone has to look at whatever you produce and give you some idea about its correctness and, hopefully, its implications. This happens in most face-to-face courses, though sometimes ineffectively, if the prof focuses too much on grading rather than more effective feedback.
If my understanding of OU is still accurate, then note that it isn't just an online information broadcaster. In addition to extremely well produced material they depend (or did, at least) on students having contact with local tutors to ask questions and such. Thus, the system there is designed to give you good information, as well as to provide you a way to do the reinforcement (exercises) and get feedback (comments, questions, ...).
For a self learner who doesn't have a guide about both what and how to learn it is a bit harder. In a purely online "course" the information may be available, but there may be nothing that forces you into reinforcement mode and nothing at all to give you feedback. The self-learner has to organize those on their own. I think OU is pretty good about that, but it takes your own dedication to make it work.
Caveat. I'm not, and never was, associated with OU, but have worked with some faculty there on unrelated projects in the past. My information isn't especially current.