Expertflyer gives fare information. Now if you actually buy a O or Q fare then checkmytrip will give the fare basis, then you can use Expertflyer to identify that fare basis, swap the selling class from O to N, and it may even be obvious (e.g. if the cheapest fare basis is OZ0RO there may also be NZ0RO with identical conditions). The fare shown is the base fare, so the taxes have to be added, these are usually identical between O and N.

You can also use ITA since if you find the cheapest O fare - and don't have too many options then after a page or two of O or Q fare combinations you should hit N.

But I usually can get a quick hunch on this by using the BA Find Our Cheapest Flight - this will be for O or Q. If you then look at the fares around, it which are a bit more expensive - same day, or a day or two away, then you can often get an N fare. Just click on the flight number to be sure, the Selling Class will move from O to N, with any luck. So you may find a round trip O fare at £70, and then an N fare at £99, to take a current example for OSL. That's a typical gap on a return fare from O to N, around £30 return.

Occasionally V is cheaper than N, and there are other complications, regarding one way and return fares, but that's the essence of it.