There's some confusion here with long and short plugs. The OS (and also Enya) plugs are actually a medium length plug, no one knows why those two companies make that particular length. A true short plug is one or two threads shorter than the OS.

Virtually all engines now a days take a long reach plug but getting down to around the .15 size it's worth checking the manual for the recommendation. If unsure then take off the head and screw in a plug. If it comes very near flush with the combustion chamber then that's the length to use. The only engines where you have to be especially careful about plug length (including the idle bar if it has one) is on the early baffled piston engines where some of them had the plug offset into the baffle groove in the piston (the OS Max-III is an example). Modern flat top piston engines will all have more than enough clearance for virtually any length plug.

Most 4 stroke plugs have an extended nose on them (the OS F plug in that photo has the same thread length as the A3) and this can be handy to use in an inverted engine because any slight overpriming doesn't get inside the plug cavity to put the fire out so the engine will start easier. It's better to learn not to overprime of course .

Heat range is the more important part of selecting a plug but really only if you're after the last few possible revs from the engine. For normal flying with lowish nitro fuels any medium to hot plug is fine because they'll also work well at idle.