In late-September 1872, O'Neill was among the cavalry troopers who followed Colonel Ranald Slidell Mackenzie in an expedition over the Staked Plains of Texas.

After a one-day march to reach the North Fork of the Red River, his regiment encountered large encampment of 280 Comanche warriors on September 29.

Although the cavalrymen attempted to take the enemy by surprise, the Indians' ponies stampeded at the soldiers approach alerting the camp.

Engaged in fierce combat with the Comanche, O'Neill and his company led the advance and were able to secure the camp with the loss of only one trooper killed and three others wounded.

The Mow-wi tribe of Comanche, who lost 23 braves in the battle, were humbled by this victory and surrendered at Fort Sill ending 17 years of continuous warfare.