However, as his ice time declined, he joined the North Jersey Avalanche in the Tier 1 Elite Hockey League following his junior year.

As his ice time at Delbarton waned, O'Connor quit the team his junior year and joined the North Jersey Avalanche in the Tier 1 Elite Hockey League.

After taking part in a summer showcase in July and performing well, he received enrollment offers from Division III schools and Dartmouth College.

[3] After adding a goal and two assists in the Big Green's 7–6 overtime win against the RPI Engineers, O'Connor received the ECAC's Rookie of the Week honor.

[10] On March 4, O'Connor earned his second ECAC Rookie of the Week honor after tallying three goals in a road split against Union and Rensselaer.

His goal count stood as the second-highest among all Division I first-year players in the 2018-19 season, while it also ranked as the third-highest among freshmen in the history of the program.

[14] Following an outstanding freshman season, O'Connor was invited to participate in the Boston Bruins' Development Camp in June and July.

[16] As the Big Green began to suffer from injuries, O'Connor was moved out of his original position into a center role.

[26] He had also been heavily recruited by the Boston Bruins, San Jose Sharks, Buffalo Sabres, and Anaheim Ducks.

The Ducks' vice president of hockey operations David McNab mailed him hand-written letters while the Sabres informed him that he could play on Jack Eichel's line.

[32] O'Connor made an immediate impact in the WBS lineup by scoring his first professional goal in his first game in the AHL to lead the Penguins to a 6–3 win over the Binghamton Devils.

[33] By the time O'Connor was re-assigned to the Penguins Taxi Squad on April 5, he had tallied four goals and four assists for eight points through eight games.

[34] O'Connor finished the AHL regular season with seven goals and 12 assists through 20 games but proved less successful at the NHL level.

[36] Over his first 12 games of the season, he played left wing on a line with center Brian Boyle while averaging 11:02 in ice time.

[37] On October 16, 2021, O'Connor scored his first career NHL goal to lead the Penguins to a 5–2 win over the Chicago Blackhawks.

[38] O'Connor was re-assigned to the WBS Penguins on November 14 after Sidney Crosby, Brian Dumoulin, Marcus Pettersson, and Chad Ruhwedel were activated from the NHL's COVID-19 Protocol List.

[44] As he continued to split the season between the NHL and AHL, O'Connor signed a one-year contract extension on March 12, 2022, with an average annual value of $750,000.

[45] O'Connor finished the AHL regular season with a career-high 12 goals and 20 assists for 32 points while leading the team with 0.98 points-per-game.

Appearing in 43 of the team's final 50 games of the season, O'Connor averaged 9:54 of ice time per contest while accumulating five goals and six assists.

[51] After the Penguins failed to qualify for the 2023 Stanley Cup playoffs, O'Connor was selected for the United States men's national ice hockey team to compete at the 2023 IIHF World Championship.

[57] Upon concluding the championship, O'Connor re-enrolled in Dartmouth to finish his Bachelor's degree in sociology and officially graduated on September 7, 2023.

He finally broke the streak on December 31, 2024, where he scored both Penguins goals in a 4–2 loss to the Detroit Red Wings.

On January 31, 2025, O'Connor was traded to the Vancouver Canucks along with defenseman Marcus Pettersson, in exchange for forward Danton Heinen, defenceman Vincent Desharnais, right wing prospect Melvin Fernström and a conditional 2025 first-round pick.