Let's start delving into the Omega Level Mutant (OLM) definition.
The word "power" as used in the OLM definition refers to mutant ability - these two words are often interchanged when used to term what mutants can do - which comprises scope, skill, and strength. Per the OLM definition, an OLM has an "undefinable upper limit" of scope, skill, and strength in their specific power/ability classification (e.g., telepathy, weather manipulation, telekinesis, etc.). Furthermore, this "undefinable upper limit" ensures that OLMs will continue or have the potential to grow in scope, skill, and strength indefinitely. In other words, an OLM has an "undefinable upper limit" of growth in skill, scope, and strength.
Moreover, there are gradations in the OLM category. Two or more mutants who are categorized OLM by the same power/ability classification (e.g., Jean Grey and Quentin Quire under telepathy) can alternately be at a higher level than the other in the manifestation of these three factors (i.e., again, scope, skill, and strength). Additionally, OLMs will always be at a considerably higher level than non-OLM mutants within the same power/ability classification in the manifestation of the aforementioned factors of said classification due to the undefinability of their upper limits in each.