Fira is the town on the cliff top where the cable car station is located.
I suspect, but can't be 100% sure, that if you are on an excursion the tender will take you to the bus park rather than to the cable car station at the foot of the cliff (that's what our excursion to Oia did). The bus would then drive you to Fira and from there you'd be on your own.
The bus will then have finished its job and you will have to make your own way back to ship via the cable car. The place where the return tenders leave from, right at the bottom of the cable car, is not the same place the buses leave from at the start of the excursion. There is no option to take a bus back down; everyone has to either use the cable car, ride a donkey or walk down the steps.
With an "on your own" excursion, I believe that the ticket for the cable car is included in the cost of the excursion and you will be given a voucher to hand over at the ticket office when you are ready to back down. If it is not included, you will have to buy a ticket at the ticket office, cost €6 per person.
With that said, and especially since you want to go to Oia, you're almost certainly going to be paying way over the odds for an "excursion" that doesn't even meet your needs. The only real benefit is that you get to go on an early tender without having to queue for tickets on board the ship. You may want to consider cancelling the excursion because if you are even the tiniest bit adventurous and given how long your ship is in port, you can totally do Fira and Oia by yourself for a lot less money and at your own pace. You will be given details on how, when and where to get non-excursion tender tickets on board. Those tenders go straight to the bottom of the cable car.
Here's what someone from my CC roll call on my recent Odyssey cruise did in Santorini:
"* 7:15 am down to the No.1 tender. I heard 75 onboard...took about 10 minutes to reach the Old Port.
* Took 8 minutes up the cable car to Fira. 6 Euros/person one-way. You'll have to buy tickets again going down.
* Option to walk up or ride the donkey (sorry, I didn't get the price for the donkey ride.) Walkers will go up approximately 500 big steps, winding up the hill and navigate around donkey dung.
* We walked straight to the bus station. A leisurely 10 minute walk; follow the yellow line on the sidewalk...or ask.
* We took the 8:30 am bus to Oia (eeh-yah)...sight of the world famous and highly photographed "Blue Domes". 1.60 Euros/person one-way. People huddled around waiting...then a very courteous gentleman started announcing "bus to Oia!"...a line formed and the bus soon arrived. Note: The man turned out to be the conductor. He firmly stopped those trying to cut the line. The bus was full with a few allowed to stand. The conductor collected the money and loudly announced each stop. (Best to have small bills or the exact change)
* I believe there were 5 stops, but the bus driver only stopped twice...most of the passengers were going to Oia.
* Fira to Oia was less than 30 minutes...but the spectacular scenery along the way will want you wishing the ride was longer.
* Oia...simply breathtaking. A short 5 minute walk will take you the Blue Domes...(yup, I took the time and kissed my beautiful wife.)
* We shared that iconic spot with a few others...all of us mesmerized...then the crowds came.
* We took the 10am bus (left at 10:15) back to Fira. This time the bus had plenty of seats...and no stops. Took only about 20 minutes to get back to Fira.
* Walked around Fira and took in more wonderful views🙂...be careful, lots of steps. Personally, the views were magnified and enhanced by the incredible aroma coming from all the restaurants.
* Back down the cable car...on to the tender...and back on the Odyssey...1pm."
Santorini is absolutely beautiful and Oia is well worth the visit. You can see some pics I posted of it in my recap blog from my cruise here:
Hope this helps and you have an amazing time on your cruise. 🙂
I had a cancellation for My current Oct 11th out of PR, I want to use the Lift and Shift for the same cruise in Oct 2021, but no cruises show for Sept or Oct ( under new bookings, I didnt do anything with my actual Lift&Shift yet). Does that mean there really are no cruises or are they just not showing in the new booking section yet waiting to see if they fill with other Lift and Shift's - I can only go in Oct 2021, so I cant take the Nov that does show