OHK is a shortening of the term "One Hit Kill". There are plenty of way to attain a One Hit Kill in the Halo Universe. The most obvious of them is with a Sniper Rifle. A headshot with a Sniper Rifle will result in an OHK almost every time, unless the target is in possession of a full Overshield. It will take 2 if they are.
Rocket Launchers are generally an OHK. Even the splash damage from a Rocket Launcher can sometimes be a one hit kill. Being stuck with a Plasma Grenade is a one hit kill. Splatters in vehicles are a one hit kill.
Generally though, when someone refers to something being an OHK, they are taking about headshots and usually, more specifically, about Sniper Rifles.