The coaxial escapement is an invention by George Daniels, dated 1975, not by Omega. Omega industrialized it in 1999, first on a limited De Ville series, then across their entire current production. The distinction matters: Daniels conceived the principle, Omega made it scalable. Roger W. Smith, a direct apprentice of Daniels, continues to install it on his own hand-crafted pocket and wristwatches.
The problem Daniels wanted to solve is as old as the Swiss lever escapement itself: the contact between the pallet and the escapement wheel tooth occurs with sliding that requires lubrication. Lubricant ages, deteriorates, and the movement's rate drifts over time. The coaxial escapement separates the impulse from the locking onto two concentric pallets — hence the name — drastically reducing the sliding component. The practical result is that friction decreases, lubricant is needed in smaller quantities and lasts longer, and the service interval extends significantly compared to a traditional escapement.
From the bench I have seen first-generation Co-Axial movements, caliber 2500, which after twenty years showed uniform and regular wear. In the 8800 and 9900 series calibers, where the coaxial escapement works alongside a silicon hairspring and non-magnetic components, the stability of rate over time is measurable and real, not merely claimed. The METAS Master Chronometer certification adds an external testing protocol — resistance to 15,000 gauss, declared tolerance between 0 and +5 or +6 seconds per day depending on the caliber — that makes the promise verifiable.
A note on terminology: in English "coaxial" is the standard form; the hyphen in "co-axial" is a residual variant. The geometry of the two pallets on concentric axes is what defines it. Bovet has patented a mechanism called "double coaxial seconds" for a seconds display on two concentric axes: it is a different architecture, with a different purpose, which uses the term by geometric analogy, not by technical kinship with Daniels's invention.
I cannot say with certainty when Omega stopped producing movements without Co-Axial in their current range: the transition was gradual between the late 1990s and mid-2000s, and some calibers derived from ETA base coexisted with proprietary calibers longer than official communication suggested.