O'Brien, who had been in charge of the Ireland Under 15 Boys team since 2013, moves up to become Under 17 head coach, while Crawford, who was assistant to Paul Doolin with the U19s, takes over as U18 head coach.
Both enjoyed successful playing careers before moving into coaching where they have excelled - firstly as assistant coaches and more recently as head coaches.
FAI High Performance Director Ruud Dokter feels the appointments are a big step forward in the development of underage Ireland international players.
"Colin has been an international coach with the FAI for many years and he has done great work with the Under 15s. It is now a step up to the Under 17s and we are very pleased to have him take on that role," said Dokter.
"Colin is an expert in underage international football and it is the right time for him to take on this new challenge. He knows the players, the international environment, and now he will get to experience competitive UEFA games.
"For Jim it is also an exciting opportunity. He becomes the head coach of a team that we view to be very important in our age group training programme and Jim has experience from his time with the Under 19s and the Under 18 home-based teams.
"Jim has also been a long-serving coach with the FAI and his knowledge of the international arena will be vital as he helps to prepare players for the Under 19 squad, where he worked closely with Paul Doolin.
"Both Colin and Jim have proved to be excellent coaches so it is a natural fit for them to step up to these new roles. I look forward to working with them over the coming months and years."
The backfill of Colin's role with the U15s will commence immediately.