Does Open Uni Maths allow progression into post-grad studies?

In principle yes, in practice it might vary depending on exactly what area you plan to go into. They don't start some of the "pure" topics until second year that stronger maths courses at brick unis tend to start up in first year, and they don't have as many upper level "pure" topics available throughout the course.
So if you wanted to go into e.g. a PhD in algebraic geometry it's possible the OU course may not be the strongest preparation for that as a result. Equally though you could do a masters +/- PGDip or something to pick up some "missing" areas potentially after doing an OU degree. Also, that said, there's nothing stopping you reading beyond the syllabus and OU students I think tend to be equipped to self study in that manner anyway!
However depending on whether it may not be as long as you think, it may be worth waiting if it's actually just a year or two after finishing 6th form you might need to wait? If you have ILR or are under the EU settlement scheme (I think) you would be entitled to home fees if you have been resident in the UK for 3 years before your course starts. So unless you don't have ILR yet I'm not sure why you would need to wait for 4-5 years...?
Note also that some universities have specific scholarship and bursary schemes for students with e.g. refugee status. Also some unis have specific scholarship and bursary schemes for students of certain nationalities (which can potentially cover full fees in some cases). This may facilitate studying there directly so do explore that as well.

Ex-colleague did his Maths degree with the OU and ended up doing a masters at Cambridge and his PhD there too later.

I will be starting Year 13 in September and would love to pursue a career in academia and research, particularly in maths-based subjects. Right now, I'm actually doing well (projected A*A*A at the end of my Year 12). However, I won't qualify for home fees nor sfe by the time I'm off to uni. And I'm really don't want to wait till I do, which would be in 4-5 years time. I won't be able to afford the international fees, and Open Uni is really my only option to pursue an undergraduate degree.
I'm great at self-studying, and I like how flexible Open Uni is. However, I'm worried that an Open Uni maths degree might close of roads to academia research in prestigious unis, which is ideally, where I want to be, as by the time I'm done with my undergraduate, I will meet the criteria for post-grad funding.
Is there anyone who's doing/did Maths with Open Uni, and how did you find the course? How's the progression career-wise and what kind of support do you get for that in Open Uni?

I will be starting Year 13 in September and would love to pursue a career in academia and research, particularly in maths-based subjects. Right now, I'm actually doing well (projected A*A*A at the end of my Year 12). However, I won't qualify for home fees nor sfe by the time I'm off to uni. And I'm really don't want to wait till I do, which would be in 4-5 years time. I won't be able to afford the international fees, and Open Uni is really my only option to pursue an undergraduate degree.

I'm great at self-studying, and I like how flexible Open Uni is. However, I'm worried that an Open Uni maths degree might close of roads to academia research in prestigious unis, which is ideally, where I want to be, as by the time I'm done with my undergraduate, I will meet the criteria for post-grad funding.

Is there anyone who's doing/did Maths with Open Uni, and how did you find the course? How's the progression career-wise and what kind of support do you get for that in Open Uni?

In principle yes, in practice it might vary depending on exactly what area you plan to go into. They don't start some of the "pure" topics until second year that stronger maths courses at brick unis tend to start up in first year, and they don't have as many upper level "pure" topics available throughout the course.
So if you wanted to go into e.g. a PhD in algebraic geometry it's possible the OU course may not be the strongest preparation for that as a result. Equally though you could do a masters +/- PGDip or something to pick up some "missing" areas potentially after doing an OU degree. Also, that said, there's nothing stopping you reading beyond the syllabus and OU students I think tend to be equipped to self study in that manner anyway!
However depending on whether it may not be as long as you think, it may be worth waiting if it's actually just a year or two after finishing 6th form you might need to wait? If you have ILR or are under the EU settlement scheme (I think) you would be entitled to home fees if you have been resident in the UK for 3 years before your course starts. So unless you don't have ILR yet I'm not sure why you would need to wait for 4-5 years...?
Note also that some universities have specific scholarship and bursary schemes for students with e.g. refugee status. Also some unis have specific scholarship and bursary schemes for students of certain nationalities (which can potentially cover full fees in some cases). This may facilitate studying there directly so do explore that as well.

Hi! I'm a current student at the OU, currently pursuing the 3rd year of the Math Bachelors (M303, M337, and MST368), and I've gotten distinctions at the OU so far.

Supposing good (Distinction grades) for my next year (3rd year), may I ask, what universities do you think would be good for a Masters in Mathematics (targetting a PhD in Alg Geom), for an OU graduate? For example, Dream schools, Reach schools, Reasonable / Match schools, Safety schools, etc.

I was thinking of kinda aiming high:
1) Uni of Toronto's MSc in Math;
2) Warwick's PGDipl + MSc;
3) Uni of Melbourne's MSc in Math / GDip + MSc;
4) Uni of Illinois - Urbana Champagne;
5) National University of Singapore

I think these are the dreams / reach, imo, haha. But idk what are the match schools - but then again tbh, we should try to aim higher to get a good profile for a PhD application, right? Coz if not, we can always turn to industry right after the Bachelor's.

I was thinking about universities in China, too - at least they're less costly, and their rankings are quite high too, but I'm not that sure abt their views on the OU, my prowess at their Chinese tests, and well. If the tutors are willing to write reference letters I can't ask them to write tons of them I guess. How many universities do you think is reasonable for me to ask of them (coz they'd have to write or at least review the reference letters)?

Also, should I wait for the 3rd year to finish so they can refer to my final module grades, or should I ask them before that?

Sorry for the lots of questions, mate ><.

Warwick is excellent for maths and the PGDip route might be a good way to deepen your mathematical background in pure areas before entering the masters and going on to the PhD I would think! You might also want to explore other PGDip options as a route into competitive masters courses too, if the funding isn't an issue. I think KCL also used to offer one? Birkbeck does but I'm not sure they have the kinds of pure content you'd be looking for. In general I'm less familiar with masters level offerings (or specific strengths in departments for PhDs outside of a couple areas that are quite well known at some unis) too, but hopefully you can find your way to that level
I can't really advise too much on most of the international options as I don't know much about them at any level (UG or PG). Looking at the UIUC programme though it looks like it would be absolutely fine to go directly into from the OU programme - I think in general in the US outside of the more "elite" colleges, they start the pure stuff much later than in the UK. Would suggest aiming to do a thesis as well there if planning for a PhD, so you can demonstrate research skills.

Thanks for the reply and advice!
I'll definitely prepare an application for Warwick's PGDip!
I was worried about getting rejected at all the schools I'll be applying for, so one of my plans was to go for a PGDip / Graduate Diploma for Math at Melbourne: it's costly, but that I suppose that means they'd be more willing to admit me, haha, and their reputation's pretty strong too, or so the ranking tables say. I've also heard of people doing a UK Masters outside COWI, and then applying for Cambridge's Part III, esp since Camb says many Part III applicants hold a masters, but to be honest, while I've gotten pretty good grades for the Level 2 OU Math courses, I'm also reading through Oxford's Maths materials at https://courses.maths.ox.ac.uk, and I'm afraid, haha!
As an aside, I've just discovered that the OU's Math undergrad programme is pretty goated. I did a comparison against Toronto's MSc pre-reqs in a reddit thread (https://www.reddit.com/r/OpenUniversity/comments/1ecu4hr/comment/lf7tzvk/), and I think that the OU makes a strong attempt at covering the pre-reqs (going by syllabus). Also, the OU's external examiner for the pure math undergrad modules got his PhD from Cambridge, wew! And the OU's external examiner for the MSc math modules got her PhD at Oxford, so that's pretty impressive! Anyways I think that's alot of yapping from me.
I guess I'll email the tutors when the modules start, and ask them directly if they're willing to write reference letters / what performance they want to see before they're comfortable / how many they're willing to write, that should be a good plan.
Thanks!

Many congratulations on the progress! Here is an ex-Part III student. I would recommend that you apply to Part III this year as long as you have the time and energy. You can always reapply after your second master and it will only show them how enthusiastic you are and the progress you have made in that extra year. I would be very interested in hearing if you did any credit counting down (higher level to lower level) or used any Postgraduate credit towards your OU Math undergraduate degree.