Castadena and Gardea, both of whom earned All-Mission League accolades last season, saw the Golden Knights start the game struggling on offense.
The duo then settled things down in the second quarter, each scoring 11 first-half points to help the Golden Knights grab a 29-18 halftime advantage.
"It's the first game and there were some jitters," said Gardea, who registered four assists and four steals. "As the game progressed, we were able to get things going.
"We trust each other."
Castaneda, who made three three-pointers in the opening half, said the Golden Knights were able to regroup.
"We were able to start making some shots, and then I made those three-pointers," Castaneda said. "When I see a chance to take a three-pointer, I'm going for it."
Castaneda made a three-pointer to give the Golden Knights — who qualified for the playoffs last season — a 19-8 lead with 6:20 left in the second quarter. On St. Francis' next possession, Gardea made a three-pointer to extend the lead to 22-8.
Newbury Park (0-1) closed to within 39-29 on a basket by Josh Munk with 2:04 left in the third quarter, but the Panthers couldn't get any closer.
O'Brien said St. Francis, which received 10 points and a game-high nine rebounds from senior forward Tei Vanderford, will look for other players to support Gardea and Castaneda.
"We were good at times and we were bad at times," said O'Brien, whose team finished with 12 steals. "When we had the lead, we weren't able to expand on it.
"The key will be to see what the other guys can do."
Newbury Park got 10 points and seven rebounds from Vance Exstrom.
St. Francis will next participate in another pool-play game against neighbor Flintridge Prep at 5:30 p.m. today. Flintridge Prep captured the CIF Southern Section Division V-AA championship last season.