University oyster help Watch
The 18+ student oyster gets you discounts on things like monthly passes, but not PAYG.
If you get a 16-25 railcard, then you can get a 34% discount on off-peak PAYG, as well as your daily cap reduced to £4.90. Talk to someone at a station and they'll set it up for you.
Now you might be wondering, "well why the hell don't they just make the discount apply to the student oyster in the first place" and I'll agree with you because it's a pain in the ****ing arse, especially because you now have to spend money on a rail card that you don't ****ing need.