Remember that at this stage, it is suspected plagiarism only.
Until the evidence is laid out in front of you, it's difficult to gauge how serious this is.
I would certainly revisit the assessment to see if anything jumps out at you.
Assuming it looks OK, then I would gather together all of the materials used (notes, articles, drafts, tutor formative feedback, etc).
I would go through everything to make sure things are watertight. Ths will also refamiliarise you with the assessment which is good preparation.
Also, get hold of the OU's academic misconduct regs which will set out the procedure and the possible penalties.
It is frightening to be in the position but you must prepare as well as you can.