O'Driscoll suffered a fractured sinus after being punched in the face in a warm-up game last night and will definitely miss the tournament opener against Namibia.
Ireland had been worried their talisman would miss the whole event in France but X-rays have revealed the injury not to be as bad as first feared.
It was thought O'Driscoll may have broken his cheekbone after being struck by lock Mikaera Tewhata during the second half of Ireland's 42-6 win over Bayonne in France.
The 28-year-old was taken to hospital after leaving the field last night but Friday's news will be of major relief to coach Eddie O'Sullivan, who had admitted he was "genuinely worried".
A statement from the Irish Rugby Football Union read: "Following an X-ray and scan of Brian O'Driscoll's injury, it has been confirmed that he has sustained a fracture to his sinus and a deep laceration under his right eye.
"This is not as serious as initially feared and the injury will not require any surgery.
"The recovery period for the injury is three to four weeks, which will rule O'Driscoll out of the warm-up game against Italy next Friday (August 24) and Ireland's first Rugby World Cup pool game against Namibia (September 9)."
Losing O'Driscoll for the World Cup would have been a major blow for Ireland. The Leinster back is rated as one of the best centres in the world as well as his country's most influential player.
It would also have meant more bad luck for O'Driscoll following his unhappy tour with the British and Irish Lions two years ago.
O'Driscoll was made captain for the Test series against New Zealand but had a shoulder dislocated in the opening 90 seconds of the first Test.
Further good news for Ireland is that the shoulder injury sustained by prop Denis Leamy last night is not considered serious.
Ireland won a physical encounter, which saw five players yellow-carded, thanks largely to a hat-trick of tries from Denis Hickie and two from Paul O'Connell.