Sergeant Tony Woodbridge said police suspected the youths started a fire in a wooden shed behind Taste Cafe, in Thames St, and a fire in a building at the town’s vacant former hospital site, in Joy St.
Sgt Woodbridge said police were informed of the alleged arson about 3pm by members of the public.
He said the fire at the former hospital was substantial, covering all three storeys of the building and a boiler room.
"Luckily, it is an abandoned building but it is somewhere where people do hang around when they shouldn’t."
Smoke from the fire could be seen from all around Oamaru.
In Thames St, a small explosion, believed to be caused by an aerosol can, was heard from the wooden shed, Sgt Woodbridge said.
Police had the three young people in their custody and were dealing with their parents about 4.30pm, he said.
The youths were being interviewed about 6.30pm and the matter was being referred to Youth Aid.
Despite the boys’ young age, the matter could end up going to court, he said.
The police had to engage with Oranga Tamariki and keep their parents and caregivers updated, Sgt Woodbridge said.
There had been no injuries.
Fire Safety and the Criminal Investigation Branch would investigate the scene today, Sgt Woodbridge said.
