O'Loughlin, who plays Steve McGarrett, has taken a brief hiatus from the CBS drama due to a stint in rehab.
"It was a different experience, but we definitely missed Alex," Oka told The Hollywood Reporter. "A lot of the script had to change, because this was a sudden development for us."
The former Heroes actor added that the remaining cast members formed a "strong bond" in O'Loughlin's absence.
"Having the smaller group, there were fewer people to share the screen time, so we actually spent more time with each other, because everybody carried a little bit more weight now," he explained.
Oka also dropped hints about the show's second season finale, describing the upcoming instalment as "amazing".
"I have to give credit to the writers that they know how to write an ending," he said. "There is going to be a big cliffhanger that's going to... make people want to come back for season three, so there's definitely that."
Hawaii Five-0 was picked up for a third season earlier this month as part of an 18-show renewal by CBS.
Michelle Borth, who plays McGarrett's love interest Catherine Rollins, has been promoted to series regular status for the new season.
> Hawaii Five-0 star Lauren German joins Chicago Fire pilot
Watch a recent Hawaii Five-0 clip below: