Ah, a topic close to my heart. What you considered originally - f/t job and f/t OU - is the beginner distance learner's trap, and although doable (the Devil's triple: 30 creds intro module, 60 creds core module and 30 creds final module; L1/2/3, respectively; say for pure maths parts alone connects up quite nicely, but that's three exams close together unless staggered, plenty to soak up and plan, and not all later modules can be staggered for comfort) is not advisable. The OU, to its credit, has subtle pushes in its literature and advice against this route for your average 40 hour week worker (and not so subtle, studying at full tilt has to be authorised beforehand). To avoid rehashing in depth what's common knowledge on the student forums there and the OU's own study advice, I'll summarise:
*Get a taste for the format (try one 30 cred module, say M140/Basic Stats - no exam, but not too boring and gets you up and running with regards to tutors, student forums, courseware, etc; if Graphs, Networks and Design is still running, it's an L3 module you can easily do as a CS grad also, for extra initial challenge - discrete maths and elementary combinatorics, mostly algorithmic); you can also skim through some OU literature secondhand or at their regional centres, iirc, to get a better idea; do try the quizzes
*Pick the right course and path through it (quite hard, even with some years behind you, but generally: if you don't like the materials and your first serious core module, it's a hint)
*Pick your fights
*Don't cram
*You don't have as much time as you think you have
*Yet commute is not an obstacle - audio materials
*Let the specimen and past exam papers guide your focus
*The minimum pass requirement on certain modules is low, but people still miss it! (A way around this is to take modules as you see fit, but link them to a qualification later to progress; still the requirements must be met eventually.)
*Consider credit transfer for the optional first year bits, if speed is important (to review foundation level material, you'll only need Essential Maths 1/2; the optional M140 stats and random choice option 2 can be safely skipped via a transfer, unless you want to try out the OU as per point 1)
*First year is review stuff and doesn't affect your final classification for the degree, but if you can't recall most of it when L2 starts, you're in trouble (more re volume and exams rather than overall difficulty of content)
*Use your tutors; tutorials are sparse but most are readily reachable by phone and increasingly digitally
*Peer support, student societies and counselling are also available
Now, there are some reasons people do load up 120+ credits, come rain or shine:
*You need a degree quick, and largely for non-academic reasons
*You do not work full time or at all
*You're doing it for fun only, and have the money for this hobby
*You're completing a degree transferred from elsewhere or otherwise formalising knowledge that's already there (say, you'd studied maths on your own for the past X years or worked in an analytic field, which relies on non-trivial modelling).
*They think 60 creds per year is too slow, and base their time estimates on year 1 courses
*The degree shadows your job
*School leavers doing OU as their primary post-secondary education
*OU is life; that is, outside work and OU you don't do anything else for three or four years, and I mean running out on the dot from work and having OU's audio in the background as you shower and eat
If the above does not apply, don't do it at f/t pace. You'll burn out and you may even end up 'hating' mathematics, which would be a great shame, as it's a deep and enriching subject and a very creative human art!
As for going at the 90, let's look at the practical matters of that approach, given the degree pathway (assuming you aren't going Maths/Phys or Maths/Stats):
Year 1: 30/30/30/30 (or L1 60, such as S104 or its successor module, which must be counted as a 30 cred option and linked after you complete all other prerequisites for L1 - you have to call them to do this, as the system won't let you, but it's possible and legit)
Year 2: 60/60; yep, after the initial rewrite of the course, modules got merged, and this is a trend with the OU overall; to do 90 optimally, you'll need to take an L2/L1 combo - fractionally taking some courses, dropping out and completing/deferring later is an admin nightmare and is regulated tightly - you will need to present good reasons beyond wanting to take 90 creds and L2 only offering a 60/60 split.
Year 3: 60/30/30 (the difficulty here varies, from plain-sailing combinatorics, which are more applied and L2 or lower to some pretty meaty applied maths and competent delivery of essential pure material; some people manage to talk themselves into grabbing a few masters modules, but this is harder now (the department is strong in diff. geometry, stats, complex and applied complex analysis, PDEs, and likes its number theory well enough, with aforementioned combinatorics brought in by new blood in the staff roster. So unless you have L1 options still left to do on your list, taking L2/3 or L3/3 double whammy is asking for a panic attack)
Above all else, discipline and perseverance gets the prize in this game. Should you choose to climb the 'mountain' - the very best of good luck!