Ezra Forde cleverly gave the home side the lead on nine minutes and the hosts held on well until the half-time interval.
Josh Rees was given his marching orders late in the second half for a late foul on Godfrey Poku, before the defender then doubled Oxford’s lead in the last few minutes.
City boss Rod Stringer made two changes to the side that beat Poole Town 3-0 last weekend; Anthony Church and Chris Dickson missed out as Michael Spillane and Shaun Jeffers returned to the starting eleven.
The hosts opened the game the brightest, with Freddie Grant and Forde threatening down the left wing.
And the hosts’ opening pressure gave them the lead after nine minutes. Poku’s quick throw-in fell to Forde who squeezed his way into the box before lifting the ball over Ross Fitzsimons.
Chelmsford had multiple chances to draw level but came closest when Josh Hill’s through ball split the Oxford defence, only for hot-shot Louie Theophanous to see his drilled shot saved by the legs of Craig Hill.
The hosts continued to go searching for an equaliser but Robbie Willmott’s cross was blazed well over by Kane Haysman.
Oxford almost took further advantage of more poor defending by the visitors, but Jefferson Louis could only fire a strong shot into the side netting just before the interval.
Stringer replaced Kane Haysman for Chris Dickson at half-time, with the City boss looking to create more chances in the second half.
The substitution almost proved to have worked on 54 minutes. Godfrey Poku farcically gave the ball away to Robbie Willmott with a sublime cross-field pass, the City winger danced forward before cross to the far post where Dickson loomed. However, the experienced striker somehow missed the target and sent his header just wide of the post.
The visitors continued to try and create a chance but the dust-bowl that is Marsh Lane troubled the Clarets, with the ball gathering pace and proving difficult to control on a hard pitch in Oxfordshire.
Forde almost grabbed a second for the home side in the final ten minutes, but the physical striker watched as he dragged his shot just wide.
Equalising became a whole lot worse for the Clarets when Josh Rees was given a second yellow for a late challenge on Poku. The City midfielder challenged Poku in a 50-50 challenge but came in a second too late and frustratingly got his marching orders.
The home side took full advantage of their numerical advantage by sealing the victory as Poku added a second in the dying moments. Forde held the ball up after checking inside past substitute Mark Haines and the ball was scrambled over the line by the full-back.
Stringer’s side tried to conjure up a late consolation goal but were withheld by a resolute Oxford side who grabbed maximum points for the first time at home in over a month.
1. Craig Hill
2. Godfrey Poku
3. Freddie Grant
4. Guri Demuria
5 Lee Henderson
6. Jean Niate
7. Reece Fleet
8. Scott Davies
9. Jefferson Louis
10. Kaiman Anderson
11. Ezra Forde
12. George Essuman
14. Wilson Carvalho
15. Matt Peake-Punen
16. Akin Daramola
17. Connor Furlong
[b] Chelmsford City:
1. Ross Fitzsimons
2. Joshua Hill
3. Luke Daley
4. Max Cornhill (76’, off for Mark Haines)
5. Michael Spillane ©
6. Ashley Miller
7. Robbie Willmott
8. Josh Rees
9. Louie Theophanous
10. Shaun Jeffers (63’, off for Elliott Buchanan)
11. Kane Haysman (46’, off for Chris Dickson)
12. Elliott Buchanan
14. Chris Dickson
15. Daniel Blanchett
16. Mark Haines
18. Matt Young
Oxford City: None.
Chelmsford City: Ashley Miller (47’), Josh Rees (62’, 85’).
Oxford City: None.
Chelmsford City: Josh Rees (85’).
Oxford City: Ezra Forde (9’), Godfrey Poku (88’).
Chelmsford City: None.
Half Time Score: Oxford City 1-0 Chelmsford City
Full Time Score: Oxford City 2-0 Chelmsford City
Referee: M. Buonassissi