Detective Sergeant Ronald Sellon was named Tuesday to take over the helm of the town’s police force when Arthur M. O’Neill retires at the end of March, after 20 years as chief. In making the appointment, Town Manager William R. Ross said that Sellon “brings an outstanding education background, including a law degree with an emphasis on labor relations,” and also has extensive experience in his 17 years on the force. Sellon, who is in charge of the detective division, was one of four internal candidates that the town reviewed with the help of an outside consulting firm, Mark Morse and Associates. The other three candidates were: Executive Lieutenant James Seekell, Operations Lieutenant Samuel F. Thompson Jr. and Sergeant Tracey A. Juda. Sellon will oversee about 36 sworn officers, 20 reserve officers, the animal-control department, and the parking division, Ross said.