Three of the fires were in wheelie bins at Waitaki Girls', one placed against the wall of the year 12 and 13 common room, and the fourth was in firewood stored outside under stairs at the Seventh Day Adventist Church, near the school.
The fire in rubbish in one of the wheelie bins at the school went out; a second away from any buildings burnt completely; but the third spread fire up the wall of the common room and into the ceiling, causing major damage.
School principal Lynlee Smith described the fires as a "mindless act of vandalism" and "pretty heartbreaking", taking the common room out of service for about 55 senior pupils.
The building had only been repainted on the outside over Christmas and pupils would be "pretty gutted" not to have their common room.
Another common room would have to be found until the building was repaired, affecting many people.
The school was lucky more damage had not been caused by the three fires, Ms Smith said.
The Fire Service was first called out about 5am yesterday when someone using a cellphone reported the fire at the church.
It caused smoke damage to the stone exterior of the church, but was contained to the firewood under the steps leading to the main door, Oamaru fire chief Steve Couper said.
Firefighters had only just returned when the second callout came at 5.38am, someone in a car having spotted the fires at Waitaki Girls' and reported it to the fire station in person.
Two units and 10 firefighters turned out, focusing on saving the common room, where the fire had spread into the ceiling.
Smoke and access to the ceiling presented problems, but the fire was under control in about 20 minutes. Firefighters finished damping down hot spots about 7am.
Fire risk management officer Stuart Ide said the three fires at the school were caused by rubbish inside the wheelie bins being set alight.
Those fires, and three in Christchurch early on Saturday morning of which two were in rubbish bins, showed the danger of having wheelie bins with rubbish close to buildings, he said.
Oamaru Detective Sergeant Dan Keno said it was fortunate the Oamaru fires were contained and had not caused more damage. Police had recovered some items from the scene of the fires which would be sent for forensic testing.
He appealed to anyone who might have seen anything suspicious in the area about the time of the fire to contact Oamaru police or Crimewatch.
