The U's fell to a team with ten men for the second week in succession, as Walsall scored an injury time winner through Darren Byfield.
Mark Hughes was sent off for the hosts at the end of the first half but the U's couldn't capitalise after the break.
Despite creating a host of chances against the ten men, a man of the match performance from keeper Gilmartin kept the score goalless.
And with the game in added on time at the end of the ninety, the pacey Byfield broke clear to break the deadlock and claim all three points for Walsall.
After failing to win in their last four matches, the U's will be hoping for a better return in the next week, with two games at the Weston Homes Community Stadium.
Boss Aidy Boothroyd made three changes to his starting eleven for the clash at the Banks's Stadium against a midtable Walsall side.
Clive Platt returned to the starting line up, replacing Kayode Odejayi up front, while Anthony Wordsworth came in for Ian Henderson in midfield.
The other change also came in midfield, with David Fox dropping down to the bench to be replaced by John-Joe O'Toole.
Walsall made two changes themselves in a bid to end a run of four games without a win, one more match than the U's had gone since their last victory.
The game started with drizzle falling steadily and the first few minutes saw the two teams feeling their way in, without creating any chances.
Anthony Wordsworth had the first effort, six minutes in, when he got on the end of a Marc Tierney cross but the shot was blocked.
The best U's chance of the first half came three minutes later when Kevin Lisbie found himself clear of the home defence with only the keeper to beat.
Unfortunately, Gilmartin was off his line quick and managed to save Lisbie's effort, which the former Charlton forward would have been disappointed not to score from.
Gray put a weak free kick over the bar before Mattis thought he had scored in the seventeenth minute.
A cross came in from the left and Matthis' volley looked a certainty to open the scoring but Ben Williams pulled off a magnificent save to divert it away for a corner.
Kevin Lisbie showed his pace again on the half hour when he beat Gilmartin to the ball but the keeper, who seemed surprised by the U's striker's pace, then just did enough to get a touch and concede a corner.
The game exploded into life a couple of minutes from half time when Hughes was red carded for a challenge on Ashley Vincent.
The home crowd weren't happy with the decision and hurled abuse at Vincent as the U's winger waited to return after receiving treatment from physio Tony Flynn.
The half time whistle went with the score 0-0 and the opposition down to ten men - a carbon copy of the situation last weekend at Carlisle.
Aidy Boothroyd would have been impressing the need for the U's not to repeat the result from Brunton Park seven days previously.
The second half started slowly and neither side had any clear cut chances but it was the U's who were making the running.
Kayode Odejayi replaced Vincent on the hour, but had to go off minutes later as a n ankle injury saw him replaced by Steven Gillespie.
The last ten minutes saw the U's finally cause real problems for the home defence but a man oif the match performance from Gilmartin denied them.
Prutton had two efforts saved, he then provided crosses for Lisbie and Wordsworth but still the U's couldn't break the deadlock.
Lisbie's header went wide, while Wordsworth could only find the arms of the impressive GIlmartin.
As the game went into the final minute, Gillespie almost scored with a left foot volley but the shot cannoned off the outside of the post.
And the U's were punished in injury time, as Darren Byfield raced clear before slipping his shot under Williams.
Byfield maybe shouldn't have had the chance though, as the referee deemed a push on O'Toole to be fair and allowed the home side to break.
So, for the second week running, the U's were beaten by ten men and must regroup ahead of two home matches in the next week.
U's: Williams, White, Okuonghae, Heath, Tierney, Vincent (Odejayi (Gillespie)), O'Toole, Prutton, Wordsworth, Lisbie, Platt.
Subs: Cousins, Batth, Ifil, Fox, Henderson