A quick update: in the end, I wrote to Omega, and they replied the following:
"We thank you for your message and your interest in OMEGA.
Following your request, we are pleased to inform you that the crown, the bezel and the case back are made of titanium.
We hope to have been helpful to you and remain,
with kind regards,
Your OMEGA team."
So Ti it is. The only thing that has me puzzled with that reply is the reference to the case back...