"The most important thing is that reactor safety was never threatened," SSM control and protection unit head Christer Sandström told the TT news agency in early March.
Environment Minister Lena Ek meanwhile condemned the environmental activists' actions.
"It's an illegal action and it costs society," Ek told the TT news agency.
Greenpeace staged similar actions at the Swedish reactor parks Ringhals and Forsmark in 2012, which lead SMS to propose armed guards on site at the ten reactor parks across Sweden. The proposal was still under consideration when the last action took place.
Greenpeace Sweden explained the action at Oskarshamn as an attempt to highlight the dangers of ageing reactors.
"We know that ageing reactors carry serious risks," Greenpeace head Annica Jacobsson told TT.
The oldest reactor on site, the O1, was connected to the electricity grid in 1974. The second O2 reactor was plugged in two years later, in 1974.
The radiation authorities downplayed any immediate risk posed by the now decades-old reactors.
"All of the country's ten reactors have gone through or are going through large-scale modernization," SMS reactor security chief Lars Skånberg said. "We don't have any immediate safety problems today. And were we to detect such problems we'd close down the reactors immediately for repairs."