Before coming to the Lakers he spent two seasons as an assistant at Colby, where he worked with the defense and served as recruiting coordinator for the Mules.

In 2020, he helped the Lakers to a 5-0 start and a number two ranking in Division II, before having the season cut short due to the COVID-19 pandemic. O'Hagan had the Mercyhurst defense ranked eighth nationally allowing just 7.2 goals per game. From that defense two players Will Agate and James Wittmeyer advanced to the professional ranks as both were signed by the Philadelphia Barrage of Major League Lacrosse.

O'Hagan assisted with the defense and ran the box for Mercyhurst in the 2015 and 2016 seasons, before heading to Colby. During his time with the Lakers they ranked second in NCAA Division II in scoring defense in 2016 (7.14 goals allowed per game) and 2015 (6.67).

Colby's conference and overall record improved during both the 2017 and 2018 season. Its goals-against average dropped by two per game during O'Hagan's tenure.

After graduating from Canisius, O'Hagan worked two years with the Iona Preparatory School lacrosse program. He was the varsity defensive coordinator for a 15-6 squad in 2014. O'Hagan coached the junior varsity to an 18-2 record and a city championship in 2013.

O'Hagan was a three-year starter at Canisius and earned Metro Atlantic Athletic Conference First Team honors in 2012 as a defenseman.