Southampton struck first when Taylor Harwood-Bellis split the Oxford defence open with a pass that found Fellows dashing down the right-hand side.
After controlling possession, the winger then drilled a pass across the box for Larin who stretched for the ball and then guided home for his third Championship goal of the season since arriving on loan in January.
Eckert’s side then doubled their lead in the 13th minute as a short corner routine found its way to Charles who curled a right-foot effiort into the top corner of the net from outside of the box, a brilliant effort.
Oxford had penalty shouts waved away early in the second half when Flynn Downes went shoulder-to-shoulder with Stan Mills and knocked him over in the box, but referee Ben Toner was unconvinced.
The U’s had an encouraging start to the second period with chances for Cameron Brannagan and Will Lankshear well blocked by the Southampton defence.
Tottenham loanee Lankshear then had a great chance to get Oxford back into the game as he intercepted the ball and ran through on goal, but his effort was wide of the target.
Fellows struck the crossbar late on with an effort from a narrow angle as his wait for a first goal since arriving in the summer from West Bromwich Albion goes on, despite registering five assists.