The Mexican assumed the lead after the Scott McLaughlin-Romain Grosjean clash and held it until he made a very slow exit from the final corner on Lap 97.
“We did everything right today … There's just always something,” fumed O'Ward on the NBC telecast which is carried by Stan Sport.
“The boys deserved that. Yeah, Dallas [Texas Motor Speedway] is next, so we'll fight for that one and compared to where we were last year here, it's a massive step.
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“We just gave that one away. We can't have that happen any more. Like, I know we're second, but yeah…”
O'Ward failed to reach the chequered flag due to engine problems on multiple occasions last year.
Asked later if it is a team issue or a Chevrolet issues, he responded, “We need to analyse.”
Elaborating on the drama, the 23-year-old explained, “It comes up randomly. You can't really predict it.
“We just have to have a look into it and just … We ended up with some great points, we started the year like we wanted to, right?
“But, they're very valuable points at the end of the day. We just got 10 points thrown away.
“We'll have a look at all the data and stuff and then just make sure how for that not to happen and look forward to Dallas. I really like that place.”
In the other McLaren cars, Alexander Rossi finished fourth on his debut for the team, while Felix Rosenqvist was classified 19th after Lap 1 contact with Dixon which set off mayhem behind them.
Team Race Director Gavin Ward said, “Obviously, a little bit heartbreaking when you think you're going to win a race and it doesn't quite go your way at the end, but that's motor racing.
“Overall, you take the big picture view here. We came in here wanting to get some good points on the board.
“It's a long season, and the key to the season is consistency and two of our cars got right up there in the top four, so have to be super happy with that.
“Felix, his crew dug deep and got that car back out there to gain some extra points and spots. I'm proud of those guys.
“Rossi has a lot of new people on his car, his first race with the team. His finish is what we need to be doing to give him a chance for the championship.
“Pato had a great car all weekend and had a strong race, but we have a lot we need to look at on the end of the race there.”
The Texas race takes place on the weekend of April 1-2 (local time), and will be streamed live and ad-free on Stan Sport.