Okai side note
I headcanon dust and killer as alternate timelines of classic
That's the only reason they're omegas

Omega males are only considered males due to having no chestal bust, fem skeletons have an ecto body from the thighs to where the shoulders start. I didn't feel like drawing a fem skel, but their eyesockets have a slit in which resembles eyelashes.

Alpha male skels have rounded eyesockets and sharper teeth than those of the omega category.
Omega males have an ecto body from the ribcage to the start of the knees.
Their eyesockets are given a sharper edge, due to the magical energy from the feminine parts.
Two omega males are able to have a theoretical child, although alphas are unable unless a form of special magic is conducted.
There is only one category of fem skeletons
Two fem skeletons are unable to theoretically have a child unless genderbending magic (requiring alot of energy) is conducted. After the spell, one of the feminine skeletons will be an omega male.
In the future stories, it will be specified at the beginning what category of skeletons each is inside of it.