None of the three transport workers – two men and a woman – was seriously wounded in the attack.
“It was during a routine ticket check on tram number 13 to Jar that three of our employees were knifed,” said Cato Asperud, spokesman for transport firm Ruter.
Police were notified of the attack at Solli Plass by the driver of the tram at 10.30am.
“Three people have been injured, but none of the injuries are serious or life-threatening,” said police investigator Oddleif Sveinungsen to news agency NTB.
Police arrested one suspect as soon as they arrived at the scene. A second man turned himself in at the central police station at 1pm.
“We don’t know if one or both are responsible for the stabbings. We don’t yet know for certain who did what,” said police investigator Per Olav Utgård.
Police were conducting background checks on the suspects on Thursday afternoon and could not yet say whether they had previous criminal records.