Oxford made five changes to the team that went down 2-0 at home to Southend, and the youngsters brought in helped them to turn in an improved performance.
Agyemang had only been on the pitch for five minutes when struck a brilliant 65th minute shot with the outside of his right foot from the edge of the area to beat Ryan Clarke.
Clarke pulled off fine stops from Agyemang and Marquis but he was beaten again the 82nd minute, Marquis headed in Yassin Moutaouakil's right-wing cross.
Constable pulled a goal back from close range with a minute left but it was too late as Guy Whittingham's team avenged their 4-1 thumping by Oxford at Fratton Park on the opening day of the season.
- Sports & Recreation