There are 4 ships arriving that day, luckily Marella is due at 7am so should be the first. It's a tender port so I would advise getting off the ship as early as possible. Marella will announce the time tender tickets will be issued and once you have a ticket you have to wait in the theatre to be called. I'd advise to get there early. You should then be able to get the cable car without too long a queue (best if you can be at the front). There are buses to Oia, they take around half an hour. It's also best to be at Oia early as it gets very, very busy. The restaurants are very expensive.
We pre arranged a hire car in Fira and drove to Oia, had an early lunch then drove around the island which was lovely. We got back to Fira but saw that the queue for coming down the cablecar was 90 mins so we had to walk down the donkey track - not pleasant as it's very slippy with squashed donkey poo - and it took us half an hour.
If you book an excursion you usually get priority on the tenders and the buses take you up a track rather than the cablecar.
Other alternative is tender to shore and you can get a boat trip to Oia from the dock, then come back to Fira by bus. Enjoy your day whatever you decide to do