And on-loan Fulham winger Williams further increased Oxford's lead in the 31st minute with a classy chip over keeper Roche after Asa Hall's defence-splitting pass.
Chris Wilder's men were totally dominant before the break and Hall also headed against the bar at a corner.
Morecambe made a better fist of it in the second half as Jack Sampson saw a low 18-yard drive come back off the post, while Marcus Marshall and Andrew Wright forced Ryan Clarke into saves.
Despite Oxford's flying start to the season - mainly down to their red-hot away form - this was only a second win in their last 10 games at home.
- Sports & Recreation
- Barry Roche
- Michael Raynes
- Asa Hall