Producing your work in the right format
It is very unlikely that your Mac does not have sufficient processing speed and memory to cope with OU courses. Only machines over 10 years old risk falling into that category.
Any course-specific software will be Windows-only. If there is specialist software for a course supplied only by the OU, you will probably have to use Windows in some form for that part of the course. The Mac will do fine for the rest.
If the software is available more widely, there my be a version for Mac OS X, or if not, a similar application that may do the same job satisfactorily. You will have to use a bit of detective work to see how the instructions in the course material translate to the Mac application you are using. There is quite likely to be a tutor who posts help in the FAQ on the course web site. Please remember that s/he does so as a courtesy, as the OU still recommends Windows.
If you need Windows, check emulation or virtualisation solutions. The Mac Compatibility List documents the experience of students who have recently studied courses. The most recent comments will generally show that courses are less incompatible than they used to be.
If your course is not listed, post a message on the OUSA Mac General forum.
As well as any course-specific software CD, the university delivers a CD with more general internet software on it, or provides links to its site for downloading. Please see the OU Online Apps for Macs page for links to all the essentials.
Most TMAs require the solution document, your answer to the questions in the assignment, to be submitted in .doc format. This is quite feasible using a Mac.
Any special file formats will be specified by the course team, and the appropriate applications provided. This may involve using virtualisation, as discussed above.
For more information on suitable software please see the software reviews section.