The Hammers squad have been undergoing two to three training sessions a day ahead of Saturday's friendly against Stevenage.
"It's been a great week," O'Brien told the club's official website. "All the lads know we need to do the training to build up the fitness, but you want to play games as soon as possible to get back in the swing of things so we're all looking forward to Saturday.
"We've got Stevenage and Ipswich before we got to New Zealand and those games are always tough because we haven't played for six, seven, eight weeks. You need to find your feet again, and no matter what you do, playing matches is completely different from the running sessions."
West Ham will take on Tottenham Hotspur on home turf in their opening game of the new season on August 16.