After the war ended, he was appointed a judge, was elected to the U.S. House of Representatives, and became a land speculator in western New York state.

His father, Thomas Phelps, died in Oliver's first year of life, and his mother was left to raise their seventeen children.

[1] At the beginning of the Revolutionary War, Phelps joined the Continental Army and fought in the Battle of Lexington.

After the war ended, he became a prominent businessman and was elected to the Massachusetts Senate in 1785 and served on the Governor's council in 1786.

[3] The connections he established during the Revolutionary War aided his efforts in forming in 1788 a syndicate with Nathaniel Gorham, also a former member of the Federal Constitutional Convention.

[1] Massachusetts owned land west of the "Preemption Line" in New York that was disconnected geographically from the rest of the state.

New York and Massachusetts reached a compromise settling their competing claims for the region in December 1786 with the signing of the Treaty of Hartford.

[6] Phelps and Gorham wanted to buy 2,600,000-acre (11,000 km2), but the Indians refused to sell the rights to any land west of the Genesee River.

It was the site of the principal village of the Seneca Indians, burned by the whites during the war in the Sullivan Expedition.

After the purchase, Phelps returned to Suffield, Connecticut, and bought what was later named the Hatheway House from its builder Shem Burbank, who as a Tory sympathizer during the American Revolution had suffered financial difficulties afterward.

But land sales failed to raise enough capital to meet their payment requirements, and in August 1790 they sold 1,276,569-acre (5,166.09 km2) to U.S.

In August 1790, the reverses forced him to sell his Suffield home and his interest in the Hartford National Bank and Trust Co.

Together, they were the parents of a son and a daughter:[1] Purchasers of his land had continued difficulty paying off the mortgage loans which he held.