In my opinion, 1911s suck for UOC if you want to maintain the illusion of 'properly loaded' i.e. cocked and locked. As such, I've only ever OCed 1911s
For non 1911 pattern pistols, its typically a simple matter of empty mag in holstered gun. Drop empty mag on draw while simultaneously bringing up a loaded mag from the offside. Then chamber a round.
I suppose chambering a round on a 1911 where the hammer is already back is technically slightly easier (no need to overcome the mainspring resistance) but the difference would be pretty minimal.