The blaze occurred in the area of Ritson Road South and Valley Drive.
Images from the scene showed huge flames burning through the roof of the complex and thick black smoke rising.
Oshawa Fire Services tweeted that fire crews rescued two victims upon arriving at the scene, both of whom were taken to hospital by paramedics.
Fire officials later said one victim had been released from hospital while the other remained in critical but stable condition.
Residents displaced by the fire are able to use the South Oshawa Community Centre for temporary shelter, officials said.
A Durham Region Transit bus was brought in to transport those affected.
There is no word on what caused the blaze.