The Islington restaurant, which recently celebrated its tenth anniversary, will re-open at the end of the month with a daily changing menu using British produce.
The menus have been created by newly-appointed head chef Jon Rotheram who previously worked as the senior sous chef at the St John Chinatown hotel and restaurant.
"This re-launch is about what I want Fifteen to be and stand for as we jump into the next 10 years," said Oliver today.
"I'm grateful that we've been busy in the last 10 years, but I really want to focus on it being an iconic neighbourhood restaurant – a place you can drop into and eat with or without booking."
As part of the re-launch, the restaurant team are expected to hold back a number of bookings each day to attract walk-ins and cement the venue's more relaxed reputation.
With Rotheram at the helm, the new Fifteen menu will encourage diners to pick a selection of small and large plates for the table. New dishes include grilled lamb sweetbreads with yoghurt and pickles, smoked venison sausage with cress and pickled mustard seeds and chocolate and salted caramel cookie with a malt milkshake.
On the new focus, Oliver said he 'loved' Rotheram's cooking: "He cooked some of the best food I had last year and is a real rising star.
"His food has honesty, edge and attitude from his time spent at Fergus Henderson’s St. John, but now he’s running the kitchen at Fifteen he's free to let his own style come through. The plates are ballsy yet elegant and precise, and comfort food at its best. On top of that, all the dishes will be totally produce-driven and great value."
Rotheram said: "I want Fifteen to be a relaxed, fun, local restaurant where you can pop in and just have a good time. For me the menu is the kind of food Jamie and I want to eat when we’re at our happiest.
"I'm going to use British produce wherever possible, and whole animals and fish will be butchered and prepared on site every day. I want the ingredients so good they speak for themselves."
Oliver originally founded Fifteen in 2002 with a view to creating a non-profit restaurant to give unemployed young people a unique chef apprenticeship. Since then the restaurant has spawned two sister venues in Plymouth and Amsterdam.
The original restaurant, which is now run under Oliver's Better Food Foundation UK charity, will feature a new bar area when it reopens. The bar will serve craft beers, wines, spirits and cocktails alongside snacks such as beef and barley buns with horseradish and cured pig's cheek with walnut.
Jamie Oliver is currently recruiting for a variety of roles ahead of the Fifteen relaunch - you can find out more and apply by visiting BigHospitality jobs here.