I studied with the OU and used OpenOffice for Mac, it's free and better that MS Office for Mac ( I have both and preferred OpenOffice). If you are studying psychology you will use SPSS software and im not sure that's Mac compatable.
MS Word on a Mac should be fine - although for the life of me I don't understand why assignments aren't submitted in PDF these days so you can guarantee how it will be viewed on the other screen. But ho hum.
If your docs are going to be mostly text (not diagrams and stuff) with footnotes I'd actually give something like Google docs a try. It doesn't do brilliantly with footnotes in documents imported from Word but it's free - and I think it works better the other way (or indeed you can save as PDF).
Well I still have my windows laptop. But my toddler got a hold of it and many of the keys are missing! So I can still view any of the software for my psychology stuff. It's just for typing things up, I was going to get mac office but wasn't sure if i submitted word docs from a mac to a windows pc it would open properly! Never had any experiance of apple computers!
Older versions of software such as Focus or Neural Networks for example do not work on MACS without a parallel operating system. Most materials, DVDs etc. are fine but it is worth checking with the OU computing help desk or faculty office