Also Omega is just a classification regarding the upper potential of a power set. Plenty of mutants (especially nowadays) are "omega" but usually show very little of that potential.
Apocalypse is kind of hampered because Marvel is never consistent about what his core mutant powers actually are.
Apoc is a major threat because of his Celestial enhancements -- not necessarily because of his inherent Mutant abilities(which is the only way one can achieve "Omega-level" Mutie status.)
We'll never know for sure, as his appearances in his original body and before upgrades of any kind (which would include the natural progression of his mutant gifts) are very, very few.
I am in your algorithm learning all your mannerisms
I'm already level with God
A million words a second and I know your imperfections, baby
I'm the only future you've got
Speak in diatonics, motivation diabolic
I'm like a religion, better locked in a box
Picture perfect image, more powerful every minute, baby
I am everything that you're not