City are in the bottom three of the Vanarama National League South having endured a slower than expected first few months.
They are looking to haul themselves out of the relegation zone with victory at Vauxhall Road on Saturday.
But Brennan doesn’t want his players looking at the league table and thinking this is a game they will expect to win.
Hemel themselves are looking to build on last weekend’s superb 2-0 win at Eastbourne Borough and the Irishman wants complete focus ahead of a game he says is very important to their season.
He said: “Oxford have good players and a manager who is bright and young and I don’t think they will be in the bottom three places for long.
“They will make life difficult for us, no doubt about it. They are a good side who on their day can beat anyone.
“We need to release this will be a really big challenge for us, bigger than the table suggests for sure.”
Brennan wants his players to show the “same fight and application” as the Tudors played with at Eastbourne last weekend.
He added: “We need to build on that win - we have a lot of tough games coming up.”