O'Hara, who has commited himself to Spurs until 2013, has been at White Hart Lane since 2005 and has spent short loan spells at Chesterfield and Millwall.
He joins Pompey on loan until January, with an option to extend it until the end of the season.
The 22-year-old has played three times for Spurs so far this season, twice as a late substitute before starting and scoring in their 5-1 Carling Cup win over Doncaster Rovers in midweek.
"It was frustrating to be sat on the bench at Tottenham," said O'Hara. "I hope I can come here and get some games under my belt.
"It's a chance to play for a side who have done really well over the last few years, so I jumped at the opportunity.
"I'm always going to give 100 per cent and hope I'll fit in straightaway and get to know the lads quite quickly."
portsmouth signing loan players.... not buying players?