Hundreds of Oklahoma inmates who served time for low-level, non-violent crimes have walked out of prison on in the largest single-day commutation of criminal sentences in US history, Oklahoma Governor Kevin Stitt said.

The Oklahoma Department of Corrections has released 462 inmates after the Oklahoma Pardon and Parole Board voted unanimously on Friday to recommended commutation of their sentences for crimes that would no longer be considered felonies if charged today.

"This event is another mark on our historic timeline as we move the needle in criminal justice reform," Governor Stitt said in a statement after he signed off on the recommendations.

Officials in the governor's office were not immediately available for comment on when the inmates were to be released.

Oklahoma's parole board, under a new state law that recently went into effect, accelerated the commutation review.

The inmates served three years on average, and will get out of prison 16 months before the completion of their sentences, saving Oklahoma $12 million (17.4 million AUD), the governor said.

In recent years federal and state authorities across the United States have initiated policies to reduce the penalties for low-level, non-violent crimes and grant clemency for inmates who served time for those types of offenses.

In 2016, voters in Oklahoma passed criminal justice reforms that made simple drug possession a misdemeanor instead of a felony. The reforms also increased the minimum dollar amount for a felony property crime from $500 (727 AUD) to $1000 (1453 AUD), the governor’s office said.

During his last day in office, U.S. President Barack Obama commuted the prison sentences of 330 federal inmates, particularly drug offenders. At the time it was the most commuted sentences in a single day, the White House said, and brought the total number of sentences reduced by Obama to 1,715.

The Oklahoma parole board considered the cases of 814 inmates and recommended commutation for 527 inmates. Sixty-five of them will remain in prison because of warrants against them in other cases.