Recent intelligence suggests that he had become a prime target for gangland criminals over the payment of a drug debt.
Gardai are paying particularly focus to a well-known dissident figure -- who is believed to have had a "major bust up" with O'Toole before Christmas.
The man, who is in his forties, is "highly dangerous" and is a known associate of murdered terror boss Alan Ryan. He is responsible for some of the most vicious assaults the country has seen in the last few years and is regarded as one of the most feared criminals in Wicklow.
"There was a major bust-up between O'Toole and this man. At one stage they were very close but for whatever reason something serious has come between them," a source said.
Officers also believe O'Toole was involved in a feud with a convicted killer who is likely to be questioned in connection with his disappearance.
O'Toole went missing from his home town of Bray, Co Wicklow, last Monday week. Gardai are confident he was abducted by rival criminals.
He has a series of convictions for gun possession and drug dealing and served a four-year sentence after being caught with a sawn-off shotgun.
O'Toole had been told in recent months that his life was under threat -- and even survived a gun attack in Enniskerry in August 2011.
"Philly knew he was in deep and had fallen out with some very serious players recently. He moved around the county regularly because he knew he was a target if he remained in Bray," a source said.
Anybody with information can contact Bray Garda Station on 01 666 5300.