The abbreviation KO (also K.O.) stands for knock out, and has its origin in the sport of boxing. Interestingly, in colloquial German, KO is often used in phrases such as
Ich bin ein wenig KO (I am a bit exhausted)
Er ist ganz schön KO (He is quite exhausted)
However, I do not remember hearing or seeing a similar use of KO in English, for instance in the form of
I feel really KO today
Instead, words such as tired, exhausted, or knackered are commonly used.
First, it is not clear if there are different degrees of being knocked out. How does a bit knocked out feel compared to just knocked out? At least in boxing, to be knocked out is an absolute and final state. The original meaning of KO may have been blurred when it became part of the German language, thus explaining the above German phrases.
Second, in English, the abbreviation KO is almost exclusively used in written English, and even then it is often spelled out (for example, knock-out price rather than KO price). In contrast, in German, KO is pronounced just like the letters K and O.
Finally, another very common term, namely OK, is used in English and German in very similar ways:
Das ist OK (That is OK)