Searching for this phrase turns up very little, and in terms of any kind of hard evidence of an "origin" is, as far as I can tell, non-existent.
The Pokemon Dictionary is the primary resource that comes up when searching for this specifically:
Short for "One-Hit Knockout", though it can also refer to moves that KO the opponent in one hit, such as Sheer Cold and Horn Drill.
Magnezone can OHKO Gyarados with Thunderbolt.
The only other source I could find that uses the phrase is Dark Souls 3, which uses it in the same manner.
The only other source I could find was the wiktionary, which only has a description, but of the three is the only one with a date:
This page was last edited on 30 September 2019, at 10:46.
Since this was derived from the concept of a "One Hit" defeat, there is a fandom collection of other tv shows, comics, animes, etc that use variations of the phrase, but this does not seem to be any kind of official source for the phrase (i.e. it is a fandom wiki), and the anagram "OHKO" is used fairly liberally throughout the page.
In terms of the origin of the phrase of "One-Hit Knock Out", this does orginate from boxing and fighting in real life - being derived from the "Sucker Punch"
A sucker punch (American English) (also known as a dog shot, coward punch, king hit or one-punch attack (Australian English) or cold-cock (American English)) is a punch made without warning or while the recipient is distracted, allowing no time for preparation or defense on the part of the recipient. The term is generally used in situations where the way in which the punch has been delivered is considered unfair or unethical, and is done using deception or distraction, hence the term 'sucker' used to refer to the victim.
In boxing, a sucker punch—as is done when 'hitting on the break', for example—is illegal. For example, when James Butler knocked Richard Grant unconscious after losing a fight to him on points, his license was suspended. It is often thrown from behind—such as in the 'knockout game'—although striking from behind is not a prerequisite for a sucker punch.