Mekki lofted a fine right-footed shot over U's keeper Ryan Clarke five minutes from time to cancel out Adam Chapman's first-half goal.
Holdsworth said: "You want all your subs to make an impact, and I was delighted with Mekki.
"He's been injured and a bit out of form but that showed what he can do.
"Our performance was very, very pleasing and that's six unbeaten on the road now, which is very good.
"I thought we were the better side and played some really good stuff.
"We had one moment of leaving someone unmarked which we were punished for, but at half-time we regrouped and bossed the second half.
"Oxford made their changes very early, and that's testament to the way we were playing."
Holdsworth felt a point was the least the Shots deserved.
"I think we could have had two penalties," he insisted. "The referee's consistency with bookings was very poor and that will be reflected in my marking on Monday."
- Sports & Recreation
- Adam Chapman