Hi jimbobuk,
In the past, I owned both the Omega Is and the Omega IIs together for some time. My memories of the Omega I are somewhat fading, but I do know that I completely preferred the Omega IIs to the Omega Is after listening to both for some time.
It seems as though when Stax designed the Omega Is, they were trying to have the headphone expand the headstage away from you as much as possible. While it sort of works, the Omega I's imaging capabilities suffer considerably as a result. They still image better than your average headphone, but compared to the Omega IIs, you simply get a fuzzy sense of where sounds are coming from. In comparison, the Omega II's have a much smaller headstage that very much feels contained within a certain perimeter that doesn't extend very far from the headphone, but within this headstage they are capable of providing extremely precise imaging. I felt less fatigued listening to the Omega IIs than with the Omega Is as my mind didn't have to cope with wishy washy imaging.
Tonally, I remember the Omega I's had a little more upper midrange/lower treble compared to the Omega IIs, its midrange a little drier, and the bass didn't go as deep as the Omega II's. The Omega II's have a smoother treble response, the midrange is warmer, more transparent, and more seductive to listen to, and the bass extends much lower and has considerably more heft and weight compared to the Omega I's. The Omega IIs simply have one of the best bass reponses I've heard in a headphone.
In the end, it was a no brainer for me to sell the Omega Is and keep the Omega IIs.