Welcome to the forum! As a 40-year-old travelling on your own, there are only really two Railcards you can get - the Network Railcard and the Annual Gold Card. The Network Railcard, although cheap at £30 a year, isn't of much use for your commute as it is has a minimum fare of £13 on weekdays (i.e. your fare won't be discounted to less than this). It's also not valid before 10am on weekdays. Might be worthwhile for weekend/evening trips to London etc. but otherwise not really relevant here.
An
Annual Gold Card may, however, be worth considering. It is a benefit you get if you buy an annual season ticket to/from/between stations in the Gold Card area. There is a
map here showing this area; essentially it covers most routes across London & the southeast (i.e. the same area as a Network Railcard). It also allows travel on extensions to Birmingham/Stafford/Shrewsbury, but only on Chiltern, Transport for Wales and West Midlands Trains services (i.e. not Avanti, and not on CrossCountry north of Banbury).
The cheapest journey which qualifies for a Gold Card is between Hatton and Lapworth (two adjacent stations just north of Warwick on the Chiltern route to Birmingham). This would cost £202 a year. You don't need to actually use the season ticket for its intended purpose in order to benefit from the Gold Card discount - you just need to buy it and show it when asked.
The Gold Card would provide you with a 1/3 discount on most fares after 09:30 on weekdays. You haven't told us exact when your "off-peak" commute is, but if it's after 09:30 then you could use the Gold Card to get this discount on your Off-Peak fare into work. The Off-Peak ticket for this journey is valid from 08:45, so if you travel earlier between 08:45 and 09:30 you wouldn't be able to use the Gold Card.
Assuming 4 days a week with a typical annual leave allowance, you'd be making about 180 journeys a year. At £8.30 a day you're currently looking at paying £1494 a year - but with the Gold Card you'd be paying £5.50 a day, so you'd instead pay £990 a year. Even after the £202 cost of the Gold Card, you'd be "quids in" after less than half a year - and that's apart from the discount you get on any other off-peak journeys to/from London, Birmingham, Reading etc. (plus the discount also extends to a maximum of 3 accompanying adults and 4 children).
Worth noting that if your commute should change mid-way through the year such that it's no longer worthwhile keeping the Gold Card, you can refund it less a £10 admin fee. The refund will be calculated based on the cost of daily, weekly or monthly/custom-period season tickets for the length you've held it. So if you refund it half-way through the year you'd get a refund of £202 less £117.60 for a 6-month season less £10 fee = £74.40. Not quite pro-rata but at least you do get a fair proportion back. Note that as annual season tickets are discounted to cost only 10.15x a monthly season, and with the £10 admin fee, there would be no refund value left after the middle of the 9th month.
Advance tickets do exist between Banbury and Oxford, but they are too expensive to be worthwhile. They are also not discounted by either a Network Railcard or Gold Card. You can also buy a season ticket or Flexi Season - but as you commute off-peak and only 4 days a week, neither are worthwhile buying.
