There’s a lot of online reading to do at the OU, and an increasing number of modules are taught only online. You need to make the process comfortable for yourself.
With ebooks and some web pages you have a degree of control over the look and feel of the page so you can improve legibility for an easier read. The crucial relationship is between the size of the type, the width of the page and the interline spacing. A small type size, a wide page and closely spaced lines make for difficult reading (for sound reasons of the physiology of the eye and related muscles – it’s not a matter of personal preference, and it affects people who don’t wear glasses as much as those who do). Adjust any or all of these and you will improve the legibility of the text.
If you’re presented with a text with these cramped characteristics the first thing to do is to increase the type size by a couple of sizes, sometimes much more. This may be all that is required to make a text readable as the lines have fewer words and the larger type has larger interline spacing by default.