Short for "One - Hit Kill". This refers to a power or attack that can defeat an enemy with just one use. It's somewhat of a misnomer, since a OHK doesn't necessarily have to kill the enemy, just incapacitate/damage them enough to win a fight under OBD rules. It is sometimes used as a verb, for example, "Gog OHK'd Superman in The Kingdom
OHK powers are generally considered to be broken.
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