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In Belfast on Thursday, the appeal court upheld the 2009 ruling against McKevitt and Liam Campbell.
The judge directed a civil retrial of the claims against Colm Murphy and Seamus Daly's appeal has been upheld.
In 2009, a judge found the four men liable, awarding 12 relatives a total of £1.6m damages.
Twenty-nine people and unborn twins died in the bomb.
Lawyers for the families had also appealed the compensation awarded. They said it should have been more because of the scale of the outrage.
In court on Thursday, the 12 relatives who took the 2009 case, were told that the £1.6m figure awarded to them would not be increased.
Lord Justice Malachy Higgins directed a retrial of the claims against Murphy and will hear arguments on a retrial of Daly.
He questioned evidence surrounding emails from a US undercover agent while overturning the judgment on Murphy.
"The paucity of the email evidence, the lack of consistency in the emails or at least ambiguity, the possibility of initials referring to someone other than Murphy and the fact that they refer on occasions to double hearsay considerably weakened the emails as evidence," he said.
"The judge's conclusion that it was cogent evidence is not sustainable."
On Daly, he said the appeal judges were not convinced that the trial judge would inevitably have reached the same conclusion about liability if a misdirection had not been heard.
"Accordingly, his appeal will be allowed."
No-one has ever been convicted in a criminal court of causing the deaths of the Omagh victims.
'Serious injury or death'
The only man to face criminal charges over the Omagh killings, Sean Hoey from Jonesborough in south Armagh, was acquitted in 2007.
None of the men being sued has the capacity to pay out any kind of large-scale payment.
From the start, the families made clear the civil action was a vehicle for putting as much information as possible into the public domain about the bombing and the men they claim were involved.
In his ruling in 2009, Mr Justice Morgan also found the dissident republican organisation the Real IRA liable for the bomb.
He said it was clear that the bombers' primary objective was to ensure that the bomb exploded without detection, and the safety of those members of the public in Omagh town centre was at best a secondary consideration.
He said he was "satisfied that those involved in the planning, preparation, planting and detonation of the bomb recognised the likelihood of serious injury or death from its detonation but decided to take that risk".
Michael Gallagher, whose son Aidan was killed in the Omagh bombing, said: "We are disappointed, we have to accept the ruling of the court, which we do, but we are disappointed.
"It has been a long struggle for the families, almost 10 years, it looks like this work will continue for a number of years forward.
"It is not something that we look forward to, but if we feel it is the right thing to do we will continue to do that."