I have used the 16-25 Railcard discount back in the day. It is an excellent way to reduce outgoings, but is not widely publicised. First buy the Railcard (online, but I recommend getting a physical card sent out) – quite often you will find 10% off codes online, then go to an Overground or National Rail ticket office and on the basis of presenting your railcard have them apply the 30% discount to your Oyster card.
Note that this doesn’t work with contactless, and you will have to remember to do this every time you renew your railcard. The 30% off applies only off-peak (not to journeys where you touch IN 0630-0930 & 1600-1900) – the touch OUT time does not matter as long as you comply with Maximum Journey Times. Sadly TfL removed the concept of the off-peak cap some time ago, so your daily cap is unaffected by the Railcard (but your actual off-peak fares are cheaper).