Ignatov pleaded guilty to several charges, including money laundering and fraud.
OneCoin and the disappearance of its co-founder, Dr Ruja Ignatova, have been the subject of a BBC Sounds podcast.
The Bulgaria-based organisation behind OneCoin Ltd continues to operate and denies all wrongdoing.
He signed a plea deal document on 4 October – but it was made public, on 12 November, only after being obtained by Inner City Press journalist Matthew Russell Lee.
By complying with the terms of the plea deal, Ignatov will not face further criminal charges for his role in OneCoin, other than any criminal tax violations that may emerge.
The document says Ignatov’s cooperation with the authorities may “reveal activities of individuals who might use violence” against him or his family.
It suggests he could apply to the US witness protection scheme and be assigned a new identity.
He has not yet been sentenced but faces up to 90 years in prison for his role in the fraud.
Ignatov has also been testifying in the trial of a US lawyer accused of laundering some of the proceeds from OneCoin.
Mark Scott is accused of routing approximately $400m (£310m) out of the US while trying to conceal the true ownership and source of the funds.
He faces one charge of conspiracy to commit money laundering and another to commit bank fraud. He has pleaded not guilty.
During that trial, Ignatov has revealed more details about the disappearance of his sister Dr Ruja Ignatova.
He confirmed that Dr Ignatova had told him critics of OneCoin were “haters”, and that she was “very tired”.
She was afraid somebody close to her was going to give her up to the the FBI, he said.
He claims she told him she had got hold of a “big passport” and asked him to get her plane tickets to Vienna, Austria and then Athens, Greece.
Ignatov said he had not spoken to his sister since she had disappeared. He had hired a private investigator to find her but had been unsuccessful.
The US government has said it expects Mr Scott’s trial to conclude next week.