does anyone have experience with the OIA?

Have you totally exhausted all components of your uni's formal complaints process? (Initial formal complaint and then review of the complaint?) You have to have done that and got a specific case number, in order to take it to the OIA (unless things have radically changed recently!).
I had a positive experience of taking two related complaints (one individual disability discrimination complaint, that only concerned me, and a joint complaint with two other students) to the OIA in 2020. Due to the severity of the case, we were assigned a case handler (the same one for both complaints) within a matter of weeks. The joint complaint was eventually deemed Justified (so uni had to pay each of us three a sum as ordered by the OIA), and the OIA helped me to settle my individual complaint with my uni for a separate lump sum
That said, I'm the only person I know of (IRL and on TSR) whose OIA complaints were successfully settled/upheld

Have you totally exhausted all components of your uni's formal complaints process? (Initial formal complaint and then review of the complaint?) You have to have done that and got a specific case number, in order to take it to the OIA (unless things have radically changed recently!).
I had a positive experience of taking two related complaints (one individual disability discrimination complaint, that only concerned me, and a joint complaint with two other students) to the OIA in 2020. Due to the severity of the case, we were assigned a case handler (the same one for both complaints) within a matter of weeks. The joint complaint was eventually deemed Justified (so uni had to pay each of us three a sum as ordered by the OIA), and the OIA helped me to settle my individual complaint with my uni for a separate lump sum
That said, I'm the only person I know of (IRL and on TSR) whose OIA complaints were successfully settled/upheld

I have exhausted it. My old university keeps emphasing my reasoning isnt good enough and keeps disregarding my panic attack evidence and that the court proceedings isnt enough to put someone in a state of panic. In addition to this they keep emphasising that I could have submitted it under a certain deadline, however this is for people to have protected characteristics, which is unrelated to me.
Thank you for actually talking in a positive light unlike the interaction I got from reddit, but how were you able to be successfull? I just feel like the uni can argue that well they followed their procedure and my reasoning is based on what I am saying and unrelated to the facts, even though both my student union and my academic schools student cases team agrees with me.

I'm sorry your uni are being so dismissive. It is hard for me to say what would make your case a strong one for the OIA. Have you pointed out to your uni that the deadline they keep pointing to doesn't apply to you? Also, do you know if your uni/department operates a 'fit to sit' exam policy (where, by turning up to the exam and answering the questions, you are thereby automatically assenting that you were in a fit state to sit the exam)? If they do have this policy, then I'm afraid it'll be very hard to argue against it - because you turned up to the exam and sat it. If there is no such policy, then you need to point to the lack of policy and say that sitting the exam did not equate to you *being in a fit state* to sit the exam and that there is no policy that says by turning up, that you *were* fit to sit the exam.
Regarding my own case: with my individual case, I think my uni knew I could sue them in a court for failure to uphold the Equality Act 2010 and that there was a good chance of me winning if I did that - so they were desperate to just pay me off, to keep it from going to court So they kept offering me different sums of money which I refused each time, because the money they were offering wasn't in line with OIA payment thresholds for 'severe distress' (which was what I had experienced). Eventually, I told the OIA casehandler what sum I'd be willing to settle for; he passed the info onto my uni, they agreed, and we settled.
With our group complaint (for which we were being guided from knowledgeable uni staff), we pointed to the fact that my uni had refused to follow usual procedures in our case; also, that conflicting narratives had been given by the uni at various points, about their actions and reasoning. Someone involved did a freedom of information act to get all the emails that had been flying around behind the scenes - so we had a wealth of evidence that uni had been irresponsible and underhand. I think that was what made us successful, though it's hard to say.
I can't quite remember when we applied to the OIA (it was during the first pandemic lockdown, I think, so maybe March or April 2020), but the money was paid to us by our uni by September 2020. So it was all over (after much back-and-forth) within 6 months or so

Yeah there is a "fit to sit" policy and they're very rigid on that. I just feel in a bit of stump because the EC i have are very justified and with evidence but its that policy that just over runs it

Ah, that does complicate the matter then. Tbh, I'm not sure there's any viable way around the 'fit to sit' policy. You could try and argue that you weren't aware of its existence but there's not really a concrete way of proving that you hadn't been told about it (either in writing or verbally) at some point...

Even outside of that my uni keeps arguing that my extenuating circumstances doesnt show why I couldn't submit it at the time, isnt that too rigid and something the OIA considers? Also, they say that it isnt strong enough to say I couldn't have submitted ECs at the time, but isnt that also way to rigid?
All I want is the chance to retake.