Present in the top two for much of the campaign, QPR are now seven points adrift of the automatic promotion spots after three defeats on the spin.
"Every game between now and the end of the season are massive," former Manchester City man Onuoha told the club's official website.
"We have reached the stage now where it is must win and this is the business end of the season.
"You can have a bad start and an iffy middle but the end is how seasons are defined and we have to step up."
QPR do have a game in hand on all the other teams in the top six and next host Leeds United on Saturday lunchtime.