Striker Marsh, who scored on his home debut last season, and Crocombe, who flies out to Turkey this month with the New Zealand Under-20 squad for the World Cup, have both signed two-year deals with the option for a further two years.
"It is great to know that the club have such confidence in us," Crocombe told the club's official website. "I am absolutely delighted to have signed a new contract at Oxford and had no hesitation at all.
"They are putting faith in young players and I think we will all be looking to repay that faith and give our all in every game."
Manager Chris Wilder added: "They are both hard-working, both have a great attitude and they have kicked on at the right time.
"They are a solid part of the first-team picture now and although they know they still have a lot to learn, we will do all we can to nurture them and help them to continue to develop."