Relatives challenged the UK authorities to "stop hiding from the truth" over alleged intelligence and investigative failings they claim allowed the bombers to perpetrate the 1998 atrocity which killed 29 people and injured hundreds, and get away with it.
They pledged to take judicial review proceedings against Northern Ireland Secretary Theresa Villiers after she ruled out a probe.
She said she did not believe there were sufficient grounds to justify a state-commissioned independent inquiry .
With the bomb having been allegedly transported to the Co Tyrone town from the Republic of Ireland, ber-eaved families have called for a cross-border inquiry.
Irish Taoiseach Enda Kenny said his Government was still considering a report the families had compiled on the alleged state failures north and south.
Stanley McCombe, whose wife Ann, 45, was killed, said: "If they want to try and hide the truth about Omagh, they can. But we'll flush them out." Kevin Skelton, whose wife Philomena, 39, was killed, said he did not believe an inquiry would "achieve anything".
No-one has been convicted of the crime but four republicans were found liable in a civil case and ordered to pay £1.6 million compensation.