The Overlord Sentry is extremely rare as it is found only two times during Duke Nukem 64: first in "Hotel Hell", spawning after the player goes to the second floor of the hotel (replacing a Battlelord Sentry in the PC version); second and lastly in the secret level Freeway, on the highway road (also replacing a Battlelord Sentry, however the Battlelord Sentry does appear in the Piece of Cake skill level while the Overlord Sentry appears in the Let's Rock skill level and above).
When Duke defeats the Overlord, Duke will say, "Die, you son of a bitch!" (censored version "Hey! You're the disease, and I'm the cure!"). Duke will sometimes say this when killing Overlord Sentries, but not always. Differently from normal enemies, the Overlord Sentry will play their normal dying animation against explosives and the like emitted from the player.
Unlike the Overlord from the PC version a Overlord Sentry cannot step on any water surfaces, and therefore cannot submerge itself. This is most likely to prevent it from falling into the sewers in Freeway.
While there are no Overlord Sentries encountered in the original PC version, there are some unused leftover data, and the port version seems to use them, like the moving speed of 84. Also, it uses an alert sound that was never used for the Overlord (b3pain04.voc); in the PC version some fanmade levels featured him but he only had 1 hitpoint since 3D Realms didn't intend for overlord sentries to be encountered at all in the PC version but they most likely considered it due to the leftover data and it should be noted various fanmade modifications have been made to allow him to take considerable punishment; notable examples include DukePlus.
one way to see the difference in size between the overord an the overlord sentry is in the chapter 2 of the alien world order episode "Red Ruckus"