We made it!
We worked with our hotel to get a taxi that knew in advance where he was going.A huge plus. We were dropped off in what seemed to be an underground parking garage with a big glass entryway. Although there was no representative from Oceania to point us in the right direction, there were security guards who asked to see our passports and boarding passes then told to proceed to the station where the porters would check our bags. To make a long long journey short, we walked down numerous hallways, up an escalator, up a ramp, across a platform all while passing through 2 security screenings and a requirement to show our passport 4 times. I did not see any motorized transporter available for anyone with mobility challenges. We were the only ship in port and I would dread to see what it would be like trying to process 10,000 passengers.
Thanks to everyone who provided input prior to our departure. It helped having some insight as to what we could expect (although 4 passport checks does seem excessive). If I visit Istanbul again it will be by air and not by sea.