Here is my rational attempt at answering the question. It depends on which definitions of these two words you choose to consider. One way I could see these expressions as being opposites would be in terms of whether you have self-control while doing these two actions. Overlooking would have a negative connotation and overseeing would have a positive connotation. "to overlook" = "to fail to notice, perceive, or consider". Ex: If you're looking for your keys on in the kitchen and you're getting upset that you can't find them, you might never find them due to your frustration. If you lose your cool, you will overlook your keys. If you had kept your cool, you would have found your keys by simply looking calmly in the room. “to oversee” = "to direct (work or workers); supervise; manage". Ex: If you maintain self-control and persevere at work regardless of the difficulties you encounter, you will be in a better position to oversee the work of others due to your ability to keep your cool. If you lose self control, you will overlook, while if you gain self control, you will oversee. I know these may be awkward scenarios but I tried :)

This is simply one of the illogical aspects of the English language!

They don't mean opposite things. They mean different things. They are different because they are not the same.