During a campaign stop in Mineral Falls, O'Rourke described the weapon used by the Robb Elementary School shooter as a weapon "originally designed for use on the battlefields" when the man started to laugh.
"It may be funny to you, m*********er, but it's not funny to me, okay?" O'Rourke responded.
The Democratic candidate received a standing ovation from his supporters.
Shortly after his speech, O'Rourke tweeted, "Nothing more serious to me than getting justice for the families in Uvalde and stopping this from ever happening again."
19 children and two teachers were killed in the Uvalde shooting in May.
O'Rourke confronted Governor Greg Abbott at a press conference following the shooting, saying the governor hadn't done enough to prevent mass shootings.
"The time to stop the next shooting is right now, and you’re doing nothing," O'Rourke said.
A recent YouGov poll shows Abbott leading O'Rourke by 5 points in November's governor race.