Both were named in the team on Tuesday to face Parramatta, but were under differing injury clouds.
Saifiti has missed the last month of football with an MCL injury and was scrambling to be back in time for the finals, while Klemmer copped a rib injury last weekend in a shock loss to the Brisbane Broncos.
Saifiti has been named to start the match, and while he may be down on match fitness, he has had a scintilating season which included playing in a winning New South Wales State of Origin series, although he missed Game 3, also due to injury.
The 25-year-old, who alongside brother Jacob re-signed at the club during the year, has made 19 appearances for the Knights, scoring three tries, making 25 tackle busts, 15 offloads and 149 metres per game.
All Run Metres
Klemmer, on the other hand, will come off the bench this weekend, with O'Brien confirming he will need a pain-killing injection to play, provided he passes the captain's run.
"We're confident that both of those guys are going to be right. They obviously need to get through that session today, but both of them looked really good the other day," O'Brien said.
"They participated in a large chunk of the session we did [mid-week]."
Klemmer has been immense in the starting side over the past month without Saifiti, playing an enormous role in ensuring the Knights would qualify for the finals.
All Run Metres
He has averaged 165 metres per game, making 15 offloads and 15 tackle busts in his 20 appearances this season.
The confirmation that the 27-year-old will play, as well as Saifiti's return, and the returns of rested stars from last week in Tyson Frizell and Jayden Brailey ensures the men from the Hunter will go into tomorrow's clash at virtually full strength.
Newcastle's season will go on the line in the match against Parramatta, who handed them a 40-4 drubbing when the sides met back in Round 13.
Kick-off is scheduled for 4:05pm (AEST) at Browne Park in Rockhampton on Sunday.