An OP Day Return A to C via B is cheaper than B to C. Can I start from B on the A to C ticket?
It's not uncommon, if there's a pricing 'cliff edge' between the fares set by one TOC and another or between various definitions of "off-peak". Whether it's allowed or not will depend on the specific ticket - for an Off-Peak Day Return ticket, on the outbound leg you are permitted to start your journey 'late'
unless the validity code of the ticket disallows this, whilst on the return leg you are always allowed (as long as it's an Off-Peak Day Return or Anytime etc.).
You can find out the validity code of your particular ticket by looking for a two-letter alphanumeric code (like 2C, 2V...) either on the ticket (if you've already bought it), or on
www.brfares.com by entering the ticket details and selecting the appropriate fare.
You can find the full meaning of the validity code at
www.nationalrail.co.uk/[two letter code], e.g.
www.nationalrail.co.uk/2V. If this says "break of journey is not permitted" (and if any conditions relating to the break of journey apply) then you can't start 'late' on the outward journey. It can still be cheaper in some cases, if you have to get a walk-up ticket for a "peak time" train, to get an anytime single outwards and an off-peak single or return to or from somewhere beyond your real destination.