That's just fine. If you are in good physical condition you can merge the sightseeing of Fira and hiking back or hike from Oia to Fira and take a look around the town.

Bear in mind that if you're doing Fira to Oia in the afternoon, you'll have the sun in your eyes and it'll be mega hot - plenty of water is essential, as well as decent sun protection.

It’s recommended to walk from Thira so as the sun is on your back, not walking into the sun.

When we did the hike we started at 8am, before it gets to hot

It's recommended to start in Fira so you don't have the sun in your eyes the whole way. Go early in the morning so you don't overheat and so the sun isn't reflecting off the water up into your face (another reason to start in Fira). The black gravel pathway will bake you. Bring water, a snack, sunglasses, a hat , sunscreen and your camera

Thanks all for your input!

If I manage to get to Fira at 8/9am, walk around town lets say until 12.00 and then I start the walk it would be the worst time? or should it be ok? Another option is to start walking at 8am to Fira, then explore fira

Starting a 3 hour hike at noon in high summer is about the worst option.

I agree with Kennydon. Unless it's unseasonably cool that day it will be very hot on that black rock trail by noon. If those are your only two options you'll be better off if you begin in Oia and start out as soon as it gets light out. Sunrise over the caldera will be very pleasant, even if it is in your eyes. Walk out on the main road towards Fira and when you get to the last bus stop on the south end of the settlement of Finikia, across from Agnadi Villas you'll see a flight of steps leading up to the cliff and the beginning of the trail. Look here: https://tinyurl.com/y2sq35oy

I was on Santorini last week and did the hike from Fira to Oia. My hotel was in Oia but everyone said to get a ride to Fira and walk back as the elevation is higher in Fira and therefore its more downhill walking toward Oia. Having finished the hike I do generally agree on the downhill slope overall, but the hike has a lot of uphill walking either way you go. From Fira to Oia the start has some up/downs on cobblestone walkways and then mostly downhill on trails after Imerovigli and not hard. The major uphill part is the peak you climb over in the last hour or so. From Oia to Fira its the opposite: a peak at the outset but then a really long, slow uphill all the way to Fira. Keep that in mind if you start after 11am, by noon it was hot.

Totally awesome hike, but I am pretty athletic and thought the 3.5 hour walk a full workout. Hot and sweaty by the end, but WOW is it a great hike.

Start the hike early, very early in July. You will beat the intense heat and it will be fairly quiet along the trail. Fira to Oia is the best option to keep the sun at your back.