I’m not aware of any new OSIs. They would need to be asked for by the RDG or DfT, and then fares set up to use them.
Looking at Slough to Staines, currently the via Windsor return fares are £14.70 and £9.20 so the half price singles should be £7.30 and £4.60. The actual single is £8.70 with no off-peak. Adding together the two PAYG single fares gives you £7.50 and £5.50. So not far out in the peak but a bit of a difference off-peak.
Other places which could make sense include Hertford and Dorking, but really anywhere with a walking interchange should be considered.