The OSCE said it did not know who was behind the attack, which consisted of thousands of simultaneous requests to load the website being launched from different locations.
OSCE media freedom representative Dunja Mijatovic called the attack "unacceptable", and said there were "forces set in motion trying to limit freedom of expression."
The organisation has emerged as a key player in attempts to resolve the crisis in Ukraine, where it has been the target of a number of kidnappings by separatists. It currently has nearly 300 civilian observers deployed in the country.