I will be in Florence with 25 friends and looking for a basic restaurant for dinner the night before an 18 mile race in the Tuscan hills. This restaurant would preferably be located in Oltrano neighborhood. We are not looking for anything fancy just want to pasta load the night before the race. If the restaurant had private rooms, it would be a huge bonus.
You'd probably be best served asking the concierge folks where you plan to stay. They'd be able to steer you. It'll also depend on the time of year you're there; during high tourist seasons there may be a dearth of options, as tour groups would be utilizing that kind of space.
We will be at a very high tourist time: end of June, 2 days before the start of the Tour de France which starts in Florence this year.
ooooof! WOW....good luck!
(I wish I could see it start in Firenze! I am seeing it finish in Nice at least!)
While not in the Oltrarno neighborhood, Rick Steves tour groups dine at Giglio Rosso restaurant. So they should be able to accommodate your group but I don't believe they have a private room.
https://www.ristorantegigliorosso.com/en/
Trattoria La Casalinga is in the Oltrarno neighborhood. I haven't eaten there but have it saved for a future trip.
The Osteria Santo Spirito has a large room in the restaurant and you can definitely carb load there. It’s very good, but well known so a reservation for 25 might be difficult unless you do it soon. They have lots of outdoor seating also, but for a group might want you to be inside. Good luck.
We ran into this situation last October with a much smaller group of 12. It was very difficult to find a place that would accommodate even that small group. With 25, it's going to be even harder. Most places we contacted said they would not accept a reservation. I would first check in to the suggestion of the suggestion above of where Rick books his groups. We ended up in a high-end hotel restaurant as they were the only ones willing to book ahead of time. Keep in mind with a large group, they will usually present you with 3 menu options, not off the menu.