The attack, which caused 28 deaths and scores of ruined lives and shattered families, will go down in history as a day of infamy. But it is clear that, far from blowing apart the peace process as the bombers hoped, it has had a bonding and strengthening effect.
Within weeks of that awful event, Unionists and republicans are publicly inching towards each other. Both sides are still mistrustful and still burdened by their own distinctive political baggage, but they are unmistakably edging towards one another rather than back into the trenches.
The Omagh effect has precedents in the South African peace process, as related by Allister Sparks in his book Tomorrow is Another Country. He wrote of one violent episode: "As with all the previous crises, this national trauma strengthened rather than weakened the political centre and spurred the negotiating parties to speed up their work."
A similar effect is visible in Belfast. The past year has been an extraordinary one, marked both by violent convulsions and political progress. The shooting of the loyalist paramilitary leader Billy Wright in the Maze prison in December last year was followed by a dozen deaths in a frenzied cycle of retaliation.
Then March saw the deaths of two friends, Protestant Philip Allen and Catholic Damien Trainor, shot by loyalists in a little pub in the Co Armagh village of Poyntzpass. July brought the Drumcree marching stand-off, culminating in the deaths of the three Quinn boys in a firebomb attack. Northern Ireland had just about absorbed that horror when the Omagh bomb went off.
In every case the immediate revulsion at the attacks was followed not by despair but by a sense of communal hope and political determination. Last autumn, most of the parties took up places round the table and refused to be dislodged by whatever violence was played out on the streets.
Easter-time brought the Good Friday Agreement; May saw a 71 per cent endorsement of the pact in a referendum; in June an assembly election delivered a similar result; July saw the first meeting of the assembly, with the Unionist leader David Trimble and Seamus Mallon of the nationalist SDLP designated respectively as First Minister and Deputy First Minister of the executive that will run the assembly.
The executive itself has still to be formed, raising the question of when and whether David Trimble can bring himself to preside over what will in effect be a cabinet containing leading members of Sinn Fein. After that will come the establishment of new links with the south and a whole series of huge questions. The issue of future policing, for example, remains to be thrashed out.
The new institutions set up under the Good Friday Agreement are obviously in their infancy, yet the sense that an important deal has been forged, and is worth preserving, has already lodged in the minds of almost everyone. More than 70 per cent of the citizens of Northern Ireland have formally endorsed it, as have almost 100 per cent of the people of the Republic.
Even at this early stage the new institutions and structures are providing a political coherence that Northern Ireland has never known before. The ultimate aims of Unionists and republicans are as far apart as ever, but the agreement has already delivered an unexpectedly firm area of common ground.
It was this new sense which made Omagh seem, in addition to being simply murderous, so politically incoherent and meaningless. And it was this sense which has made those involved in the peace process opt not for mutual recrimination but for what has been, in effect, a negotiation.
That negotiation was perhaps hastened by Omagh, but its exact timing was most of all determined by the fact that Bill Clinton's plane today touches down in Belfast. This US President has invested a lot in the peace process: this week he called in his markers, insisting he had to have something upbeat to take back to Washington.
The general communal desire for progress was thus reinforced by American insistence on flexibility, though, to be fair to David Trimble and Gerry Adams, neither seemed opposed in principle to the idea of a session of give-and-take.
Mr Adams wanted a public handshake with Mr Trimble and an assurance that the First Minister would not attempt to block Sinn Fein members taking their place on the executive. Mr Trimble wanted IRA arms decommissioning, a republican declaration to the effect that the war was over, and movement on the issue of "the disappeared".
This last refers to the dozen or more people the IRA is believed to have abducted, shot and buried in the Seventies. The plight of their families, who in the absence of their bodies have never been able to grieve properly, has been increasingly recognised in recent years.
Neither side got exactly what it wanted, but each got something. Mr Trimble has called together party representatives for a meeting on Monday. This is ostensibly to discuss how the assembly proceeds, but everyone believes it has been designed to be the first direct contact between the Unionist leader and Gerry Adams.
The two were in the same room together on many occasions in the negotiating sessions in the lead-up to the Good Friday Agreement, but did not speak to each other. This is to be the first occasion when they will have personal engagement.
On the republican side, the IRA said last week that it is taking seriously the question of the disappeared. The republicans have clearly concluded that the issue will not go away, and that the grisly business of disinterring bodies and properly laying them to rest should be tackled sooner rather than later.
On Tuesday, Mr Adams said violence must be a thing of the past, taking Sinn Fein full circle from the time when the party simply acted as propagandist cheerleaders for IRA violence. Gone are the days when the Sinn Fein paper had a column headed "War News" with the purpose of recording and commending the latest bombings and shootings.
Then, yesterday, Martin McGuinness was appointed as Sinn Fein's representative to meet the international commission on decommissioning. The arms issue is the most problematical of all, as is evident from conversations with grassroots republicans. The Adams statement gives no problems, since it is regarded as little more than a reflection of the near-universal hope and belief among republicans that the major republican and loyalist campaigns of violence are over for good.
At the same time, even the most dovish disapprove of decommissioning, partly on grounds of principle but largely because, as the events of the past year have shown, Northern Ireland is a most unpredictable political entity and no one knows what the future might bring.
So it is not clear whether there will ever be actual de-commissioning, just as it is not clear whether Mr Trimble will seek to block Sinn Fein from the executive. It is also not yet known whether the IRA will cease the savage "punishment beatings" of alleged miscreants in the republican ghettos.
Progress may be taking place, but it is not doing so on the basis of an increase in mutual trust. Rather, the sense is that things are moving along because powerful elements - London, Dublin, Washington - are there first to apply pressure and then to act as guarantors and witnesses of deals that are being worked out at one remove, since face-to-face contacts have yet to take place.
Movement is taking place on a number of inter-connected levels. There is the business of constitution-building, with the gradual construction of the assembly, the executive and crossborder links. And the question of Sinn Fein seeking to establish democratic credentials.
Then there are the legacies of all the years of violence. There is the question of how to provide better care for victims and of how to minimise their pain when they witness the release of paramilitary prisoners, as they will in the next few weeks.
To set out the many problems that lie ahead is to formulate an agenda daunting in both its size and its difficulties. There are no guarantees that all this will work and eventually deliver a settled peace.
The hope for success lies, however, in the fact that so many obstacles have already been surmounted, and that even something as vile as Omagh has not extinguished the common determination to press ahead in the cause of peace.
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