As you've asked this in the forum for the Open University, I'll assume it's a question about said university.
The OU has two main types of degrees: named degrees, like conventional universities, and Open degrees, where you make your own degree up from pretty much any collection of modules you care for. (Not all OU modules are available to an Open degree, and you do need to plan your lower level modules carefully to make sure you take appropriate prerequisite modules for the late stage modules you want to take.)
If you take a named degree course, you will have a few choices within the path of some modules, but won't be able to take anything unrelated. You'd have to change your degree programme, take the module you wanted, then switch your programme back. If not self-funding, you'd have to be cautious of funding restrictions as well (for second degrees, and such).
There's also the possibility of a dual-honours degree, where certain named degrees are paired with certain other named degrees. An example is that the OU offers Computing & IT and Psychology as a dual-honours course.