How to parent eye armature to head bone in body armature I'm following this tutorial on Youtube.

At 4:22, he somehow parents the eye armature to the head bone in the body armature. He doesn't actually show how to do it but uses an analogue by copying a bone and moving it where the head bone would be. 

I select the body armature and then shift select the eye armature and go into edit mode and follow what he is asking to the best of my ability but there is no option for "keep offset" when attempting to parent. I does not function the way it should. Is there a correct method?

Also while I'm asking, would a similar method be used for parenting say teeth to the head bone? Thanks.
Your question sounds like you have two different armatures (which are objects), you're selecting both and then entering edit mode (which in this case will be a "double" edit mode, working simultaneusly on two different objects). But the parenting you're looking for is possible only internally to one single object. 

So, select the eye armature, shift select the head armature, press Ctrl J (join two objects into a single one).

Then select the new object (armature), enter edit mode, select the eye bone, shift select the head bone and press Ctr P (keep offset).

And, yes, teeth and head can be treated in the same way (create the new bones in edit mode with shift A or Shift D).