At the
Battle of Monmouth on 28 June 1778, Olney led the two consolidated Rhode Island regiments in Varnum's brigade which was temporarily commanded by Colonel John Durkee. Part of
Charles Lee's advanced guard, this 300-man unit also included the converged 4th and 8th Connecticut Regiment plus two attached guns from Captain David Cook's company of the
3rd Continental Artillery Regiment. As the American units came into contact with the British rear guard, confused fighting broke out. Some British light dragoons attacked a party of militia horsemen but were driven off by volleys from infantry detachments under
Richard Butler and
Henry Jackson. Butler and Jackson moved forward but came under fire from two British cannons and pulled back into some woods. Thinking Butler and Jackson were enemy forces, Lee sent Durkee's soldiers to attack them. The matter was quickly straightened out and Durkee was shifted to support two guns commanded by
Eleazer Oswald on the left flank. At this time Durkee was wounded and Olney succeeded him in command of Varnum's brigade. Soon after this, Lee became aware that British commander
Henry Clinton assembled 6,000 troops nearby, many more than he had suspected. Oswald withdrew his two guns because they were out of round shot and the crews were unable to bring up their ammunition wagon. As Clinton's troops moved forward, Lee lost control of his division. Some American units began retreating and other units, seeing their fellow troops withdraw, fell back as well. Observing his troops withdrawing without orders, Lee gave orders for a general retreat. Oswald, Lee's artillery chief, massed 10 cannon to cover the movement. Pressed by British columns, the retreat continued and Oswald dissolved his large battery, returning Olney's two guns as well as two guns belonging to
William Maxwell's New Jersey Brigade. Two more guns were sent to the rear. Washington appeared on the scene and relieved Lee of his command. However, he soon relented and permitted Lee to organize a holding action while he deployed the American main body into a defensive line. Lee placed Olney's brigade behind a north-south hedgerow with two of Oswald's guns in support on their right. Oswald's other two guns took a position where they could cover the detachments of
Nathaniel Ramsey and
Walter Stewart on the left flank. Lee directed
Henry Livingston Jr. to protect Oswald's two guns on the right, but instead he took position behind the hedgerow on Olney's left. After a sharp struggle with the British foot guards, Stewart and Ramsey were driven out of the woods on the left. Both Stewart and Ramsey fell wounded and Ramsey was captured by some troopers of the
16th Light Dragoons. Next, the dragoons attacked the hedgerow but were repelled by Olney and Livingston. Clinton yelled for the 2nd Grenadier Battalion and half of the 1st Grenadier Battalion to attack the hedgerow. Charging into intense musketry and case shot, the elite grenadiers broke into the hedgerow position. Oswald pulled his guns out just in time, covered by Olney's men. In the melee, 16 grenadiers found themselves surrounded by Olney's troops, but the Americans were so bent on retreating that they paid no heed to their enemies. During the violent struggle, the commander of the 2nd Grenadiers
Henry Monckton was killed. After this action, which occurred about noon, Olney joined the rest of Lee's division which was reorganizing in the rear of Washington's main body. ==Later career==