You will have to have a reasonable UG background in maths, so it depends on what level and what type of maths course you studied. You will need advanced calculus and linear algebra as a minimum, preferably studied at Honours level, so at Level 2 or Level 3 with the OU. If you did the OU's Mathematical Modelling course, and did well in it, then that should stand you in reasonable stead, but I know that all of the OUs later-stage physics courses develop the mathematical techniques a bit more in a directed way depending on the subject you are studying.
The OU is very well established, it's been going for over 40 years as a chartered University, longer than the polytechnic conversions of the early 90s. I started my study at a traditional uni and finished it with the OU (Physical Science), and I'm starting a PhD in a crossover area between biology and physics. I did have a good maths background from my initial studies and I added to it with the OU though.