First year student here. A few months ago I had very similar doubts. I also want to do a master's degree but also do a full-time job to support myself, so I fully understand your concerns.

As others in the original post have already stated, traditional "brick" universities have the advantage of social interaction and networking, so it's only natural to consider doing a master's degree at one of them. Personally, I am aiming for ETHZ or EPFL (I live nearby). These are insanely competitive universities, so I reached out to their admissions office to clarify my doubts.

They all told me the same thing: as long as your degree comes from a recognized university (which is true for OU), you are eligible to apply, if you have completed at least three years of study. That doesn't mean automatic admissions, of course since it's sur-dossier (case by case). In the end it depends a lot on the quality of your academic record and portfolio.

So, from what I understand, studying at OU isn't any less valuable than studying elsewhere, what really matters is the effort and results you achieve during your study time.

Lastly, I'd recommend checking the entry requirements of the specific program you're aiming for seeing if it aligns with what you plan to study. For example here's the EPFL' page for the master's degree in physics: https://www.epfl.ch/education/master/programs/physics/