Esergepov, editor in chief of the "Alma-Ata Info" weekly, was arrested last week after his newspaper published leaked internal documents of the Central Asian state's National Security Committee. He is on hunger strike in custody.
The OSCE representative for the freedom of the media sent a letter to Kazakh Foreign Minister Marat Tazhin, urging Esergepov's release, the organization said in a statement.
"As the most important first step, I ask your authorities to release the journalist," the statement quoted Miklos Haraszti's letter. "Keeping him under arrest, just as threatening him with imprisonment, would be a violation of the OSCE commitment to facilitate a fearless atmosphere for public-issues journalism."
Kazakhstan will chair the OSCE next year, a decision criticized by some human rights groups. The ex-Soviet republic has never held an election judged free and fair by the OSCE.
Local media often criticize senior officials, but almost never attack President Nursultan Nazarbaev who has run the country since 1989.