Best practise is to use the largest radius your space will allow. (The absolute minimum radius for BR(SR) standard mk1 based third rail EMU's was four chains, which scales 1056mm/41.5" radius in OO; and this required the coupling and gangway systems to be specially adjusted to allow the train to creak, groan and squeak its way in and out of Southampton, on the Northam curve.)
As mentioned above RTR OO models are now made with R2 capability, 438mm radius. Longer vehicles like coaches, look better on curves as the radius increases.
The 'traditional' advice is that if a radius under 24" is required, then use set track. (In OO set track the largest radius offered is 572mm/21.5" '(R4) and this is only available in code 100 rail). However, that advice dates from the days of code 100 rail as 'standard'. Code 75 flexitrack is way easier to bend into smooth curves, and will go tighter without difficulty, I have managed scale for 1 chain - just over 10" - but strictly only for wagons in locations like docks, where a horse or road vehicle was used to move the wagons.