Marvel isn't exactly 100% consistent with what exactly qualifies someone as omega class. This eliminates a number of mutants that you might think would be classified as "Omega Level," like Magneto and Storm (or even obscure characters like Hyperstorm), who just were never officially labeled as such in the comic books.

In fairness though it doesn’t seem Marvel themselves actually stick to any one way of scaling them and, quite often, some may contradict other levels given. Omega Level Mutants are mutants whose dominant power is deemed to register, or reach, an undefinable upper limit of that power's specific classification. As far as I’m aware there’s never actually been any classification given to him. Mutant power classifications have never been fully explained.
Apparently, while Magneto is enormously powerful, his potential is limited. However, Omega Level Mutant (with or without hyphen), mutants with Omega-Level Potential, Omega Level Power, Omega-Level abilities, Omega Mutants, or simply Omega Level or Omega (also called Omega-class mutants by Xavier Institute's and the O*N*E, "Omega Levels" and "Omega strength mutant" by the Stepford Cuckoos. No. It has something to do with their potential power I believe. Magneto was previously thought of being an Alpha-mutant, but is currently considered to be an Omega Level Mutant. Omega Level Mutants are mutants whose dominant power is deemed to register, or reach, an undefinable upper limit of that power's specific classification. Marvel's Omega Level Mutants, Ranked From Least Powerful To OP.