Gen Mejia, 80, is accused of ordering massacres in indigenous Maya villages during Guatemala's 1960-96 civil war.
His lawyers say he recently suffered a stroke and is not physically or mentally capable of answering the charges.
Prosecutors dispute this, saying he was under sedation at an initial hearing.
Oscar Mejia was arrested in Guatemala City earlier this month after a warrant was issued against him on charges of genocide.
He ruled Guatemala from 1983-86 after seizing power in a coup.
He also served as an army general and defence minister during the long civil war, in which an estimated 200,000 people were killed as the army battled left-wing guerrillas.
A UN-backed truth commission found that the security forces were responsible for the vast majority of killings.
It also concluded that massacres of indigenous Maya communities accused of supporting the rebels amounted to acts of genocide.
Few senior army commanders have ever faced trial for the atrocities, but in recent months the government of President Alvaro Colom has begun to try to bring suspected war criminals to justice.
Another former general - Otto Perez Molina - is favourite to win the presidential election next month.
President Colom is not allowed to stand for a second term.

Justice Ginsburg, 80, officiates this weekend at the wedding of Michael Kaiser, 59, president of the Kennedy Center performing arts space, and economist John Roberts, 32.
Mr Kaiser is a personal friend of hers.
In June, the Supreme Court overturned a law that banned recognition of same-sex marriages by the US government.
The ruling granted legally married gay men and women access to the same federal entitlements available to opposite-sex married couples.
Justice Ginsburg, who was appointed to the nine-member high court in 1993 by President Bill Clinton, voted in the majority in that case.
Same-sex marriage is now legal in 13 states and Washington DC, and since the court's decision, the federal government under President Barack Obama has taken several steps to grant same-sex marriage couples the same legal status as opposite-sex couples.
The ceremony will take place at the Kennedy Center in Washington, the Associated Press reports.
Justice Ginsburg told the Washington Post newspaper she believed none of the Supreme Court justices had been asked previously to conduct a same-sex ceremony for fear it would compromise their ability to hear a case on the issue.

The Ministry of Justice and the Wales Office said a total of £30m will also be spent on local businesses as the new £250m prison is built.
They said £1.1m had already been committed to companies in north Wales.
Work started on the prison at Wrexham industrial estate in September and is due to be completed in 2017. With 2,000 inmates it will be the UK's largest.
It is being built on the site of the former Firestone tyre factory.
The construction company Lend Lease, which has been selected to build the prison, is holding a meeting to determine how best to deliver its promises to the community during the construction.
The commitments include 100 new apprenticeships and 500 work placement days per year.
Former Guatemalan military ruler Oscar Mejia Victores has been sent to hospital to see if he is well enough to stand trial on genocide charges.
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US Supreme Court Justice Ruth Bader Ginsburg is poised to become the first judge on the country's highest court to perform a same-sex marriage.
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The "super-prison" being built in Wrexham will bring £23m a year to the local economy and 1,000 jobs.