Academy: Watford 2-1 MK Dons
Victory puts the Hornets third in the Youth Alliance South East conference, two points and a place behind second-placed Luton Town.
Dave Hughes' side went in 1-0 at the break as Kyran Tshimanga scored from close-range.
The young Hornets quickly came close to levelling after the break when Il Myeong Choi climbed impressively to nod Kurtis Cumberbatch's corner.
Hornets 'keeper Dan Wilks did his bit for the cause, denying the Dons on several occasions.
Obi came off the bench at half-time and the forward equalised halfway through the second period when he won possession on the edge of the 18-yard box before firing into the bottom corner.
Dons goalkeeper Charlie Burns denied obi again before the forward won the game 10 minutes from end with a deflected effort.
Speaking to the Official Site afterwards, youth coach Hughes said his side had forced the issue more after the break.
"I thought we perhaps were waiting for something to happen, rather than making it happen. That was the message at half-time.
"We wanted to be in charge of what we were trying to achieve, and play a little bit more on the front-foot, and I felt we did that in the second half,' enthused Hughes.