The report is based on information from refugees in Kyrgyzstan.
The report says that Uzbek authorities repeatedly used force against unarmed civilians throughout the day of 13 May, adding that the force was used in an "indiscriminate and disproportionate" way.
It also says there are no indications that authorities warned civilians before they were fired upon. It also says that summary executions were reported.
The spokeswoman for the ODIHR, Urdur Gunnardottir, told RFE/RL today that reports strongly indicate that summary executions were carried out.
"Many reports indicate strongly that there were extra-judicial killings of unarmed civilians, summary executions, even, during these events [in Andijon]," Gunnardottir said.
It remains unclear how many people died in the Andijon unrest.
ODHIR's new report estimates that between 300 and 500 people may have been killed. The Uzbek government says 176 people died, including security forces, Islamic militants, and civilians.