Mr O'Hagan was shot yards from his Lurgan home
Mr O'Hagan, 51, was shot near his home in Lurgan at about 2230BST on Friday.
The Red Hand Defenders, a cover name used in the past by both the Ulster Defence Association and the Loyalist Volunteer Force (LVF), said they carried out the murder in a call to the BBC newsroom in Belfast on Saturday.
Dr Reid said on Saturday he was absolutely determined to track down the killers.
"It shows contempt for human life, contempt for the freedom of the press and contempt for the people of Northern Ireland," he said
"I have spoken to the chief constable and I share his absolute determination to track down those responsible for this act of savagery."
Mr O'Hagan worked in the Belfast office of the Dublin-based Sunday World newspaper where he built a reputation covering paramilitary and drugs-related stories.
He was shot just yards from his home on the Tandragee Road as he and his wife walked home from a pub.
A car pulled alongside the couple and a gunman shot Mr O'Hagan, who died at the scene.
Marie O'Hagan escaped death when her husband pushed her into a hedge to protect her.
She said on Saturday: "I can't really remember what happened.
"People tell me Martin shoved me into the hedge and used his own body to try to shield me.
"It's coming back to me in flashes but the fact that I wasn't shot dead or wounded speaks for itself."
One of his three daughters, Ciara who is due to be married in a few weeks time, was passing by in a car when she saw her father slumped on the ground.
Ceasefire doubts
The murder comes at a time when the government has the UDA ceasefire under close scrutiny.
Dr Reid warned on Friday that he would declare the ceasefire over if there was any more UDA inspired violence, following a week of serious rioting in north Belfast.
However, at the early stages of the investigation into Mr O'Hagan's murder, it is understood the main focus is on the LVF.
The journalist had recently been working on a number of stories involving LVF members.
The shooting occurred in Wheatfield Gardens, a mixed Protestant-Catholic area on the edge of the town.
A car which police believed to be the vehicle used in the killing was discovered burned out a short distance away.
The Taoiseach Bertie Ahern has also condemned the killing.
The Belfast editor of the Sunday World, Jim McDowell, said he was "devastated".
Death threats
"He was a journalist who never stood back in his life. If there were issues to be addressed, then he did it," he said.
"As a fearless journalist and a secretary of the NUJ in Belfast, an attack on someone of his stature is an attack on the freedom of speech and freedom of the press."
The chairman of the Ulster Democratic Party, John White, condemned the killing but said Mr O'Hagan had made enemies of many loyalists.
"He did cross many loyalists over the years, as did a number of Sunday World reporters, but in this day and age with the peace process at the stage it is and the difficulties going on this certainly doesn't help and there can be no justification for it."
The leader of the Ulster Unionist Party and Upper Bann MLA, David Trimble has condemned the murder.
Mr O'Hagan had received death threats before and had to leave Northern Ireland at one time.