He said that although he had attended a meeting in the capital, Libreville, between Obame's representatives and those of at least 13 other candidates, he had not agreed to any compromise.
Oye Mba told journalists that he had stayed at the meeting until midnight, but was tired from campaigning, and went to bed, only to hear in the morning that "candidates had allegedly rallied round Mba Obama."
"You can type my name on a statement, but what matters is my signature," he said.
Anna Claudine Assayi Ayo, one of the candidates who backed Obame, however, said: "The first person who voted was Casimir Oye Mba. It was he who suggested the idea of a vote!"
"Chances of opposition unity are slim, most of the alliances are ethnically oriented"
Three other presidential hopefuls - Victoire Lasseni Duboze, Bruno Ben Moubamba and Jules Aristide Bourdes Ogouliguende - also said that they had not declared for Obame.
"It's a dirty trick," Moubamba told the AFP news agency.
Vasceslas Maboundou, a political analyst, told Al Jazeera that it was always unlikely that the opposition would be able to agree on a common candidate.
"Chances of opposition unity are slim, most of the alliances are ethnically oriented," he said.
"The candidates too have parallel interests and different backgrounds."
Al Jazeera's Mohammed Adow, reporting from Libreville, said that there it would be difficult for any of the opposition candidates to achieve victory on Sunday.
|Ali Bongo has campaigned on the promise of better education, healthcare and homes [AFP]
"Ali Bongo and the governing party have been running a very good campaign. He has been campaigning not only in the capital but also in the regional areas," he said.
"The opposition remains a divided house. Out of 22 opposition candidates on the ballot papers it is only five who have said they will back Obame."
Ali Bongo has campaigned on the promise of better education, healthcare and access to new homes.
"We are together and we know that by sticking together we are going to look at the future with confidence," he told supporters.
He has even spoken out against the system of kickbacks and corruption that critics say was prevalent under the more than 40 years of his fathers's rule.
There is widespread anger, however, over Omar Bongo's huge personal fortune, accumulated while most Gabonese remained in poverty, and there are fears of violence in Libreville if Ali Bongo wins.
Marie Roger Biloa, founding editor of Africa International, said that the Gabonese have nothing against Ali Bongo as a person, but they want change.
"They want free and fair elections and they believe they won't have that," she told Al Jazeera from Paris.
"First of all being nominated by the party was not very democratic, which led to the splitting of the party with heavyweights leaving.
"Also all those who decided to run were barred from the government, with the exception of Ali Bongo."