The Pacers responded by holding New Jersey to eight field goals in the second half in a 98-80 victory Saturday night.
"We need to be aggressive in everything we do defensively," O'Brien said. "We pointed out at halftime that we were not as aggressive as we need to be. That's why they shot 50 percent. Our guys took it upon themselves to kick it into another gear, and when you play with that type of defensive disposition, good things generally happen."
The defense helped the Pacers make up for a bit of an off night from Danny Granger. The NBA's No. 2 scorer shot just 6-for-17 from the field, but made 10 of 12 free throws and scored 23 points for the Pacers (2-3).
Indiana guard T.J. Ford nearly had a triple-double, but settled for 13 points, nine rebounds and eight assists. Troy Murphy had 17 points and 10 rebounds, and Jeff Foster added 12 points and 13 rebounds for the Pacers.
But the key was the defensive effort. New Jersey shot 24 percent from the field in the second half. Vince Carter scored 31 points, but the rest of the team shot 18-for-53.
"It was outstanding, it was a good effort from us," Granger said. "Vince had 31, but we forced him into some tough shots, made it hard for everyone else on their team."
New Jersey guard Devin Harris missed the game because of a sprained left ankle. He had a career-high 38 points Friday night in New Jersey's win over Detroit. His replacement, Keyon Dooling, scored 17 points Saturday.
Ford said the Nets needed Harris.
"It changes a lot," Ford said. "Just understanding he had a big night last night and they beat the Pistons, their confidence was probably taken away a little bit knowing that he wasn't going to play."
Carter said Harris' absence didn't justify the poor effort.
"It puts more responsibility on me," he said. "But that's no excuse, either. This is a team game, and it's going to take a collective effort to win games."
The teams combined for nine 3-pointers in the first quarter, and the Pacers led 32-27 at the end of the period.
New Jersey shot 50 percent in the first half and led 51-49 at the break. Carter scored 20 points on 7-for-12 shooting in the first two quarters.
The Pacers led 53-51 when Josh Boone couldn't quite gather himself on an alley-oop attempt. Indiana rebounded and pushed the ball down the floor, and Murphy made a 3-pointer to give the Pacers a 56-51 lead.
Granger then stole the ball from Yi Jianlian at the top of the key, drove and split two defenders for a layup and a chance for a 3-point play. His free throw made it 59-51.
New Jersey's first points in the second half came with 7:40 left in the third quarter, but the Pacers maintained control. A 3-pointer by Marquis Daniels increased Indiana's lead to 66-55 midway through the period, and the Pacers led 74-66 at the end of the quarter. New Jersey shot 4-for-20 from the field in the period, and Carter was held to four points on 1-for-5 shooting.
"I forced my first couple shots in the second half, which is something I don't need to do," Carter said. "It starts with me just being a leader and taking good shots myself."
Indiana started the fourth quarter on an 8-1 run, with all its points scored by Granger, to take an 82-67 lead.
Granger said such spurts will come in O'Brien's up-tempo offensive system.
"The way we play, when one of us get going and run off six, seven points in a row, it pretty much puts the game out of reach for the other team," Granger said.
The Nets didn't make a field goal in the fourth quarter until Dooling scored with 5:55 to play. Indiana's Jarrett Jack answered with a 3-pointer to give the Pacers an 86-70 lead.
It was a disappointing effort for the Nets following such a big win against Detroit.
"They turned up the intensity," New Jersey coach Lawrence Frank said. "Our shots were poor. They beat us in all the main areas of the game, and our offensive efficiency really hurt us."