After searching around, this appears to be an issue with SlimKat ROM itself.
Reading this review of SlimKat or alternative link to article as their formatting is awful, the line that jumped out at me was:
....talk to by simply saying "Okay Google" (sometimes even just okay-_-)
suggests this happens on the ROM.
Also there is a theory that the launcher listens to OK G and starts background tasks for the searching.
Maybe it has something to do with the fact that it listens for "OK G--" and then it starts loading voice search while you're still saying "oogle", instead of it listening for "Google" and then loading up voice search, leaving you waiting for a short while?
It is worth noting that a friend with a Nexus 4 told me that sometimes it pops up on "OK" for him but it is rare. I can't find confirmation online.
Also some launchers try to incorporate the OK Google themselves, sometimes not in the correct manner and this can cause conflicts. For example the image below is from Nova Launcher (which I use myself very often) to show you its OK Google screen which is a separate function to the Google Search app's one:
Finally some OEMs have their own "Hi Galaxy" ala Samsung and this can further muddy the water with this functionality.