"At least 50 people including one (Oromia) regional official and his driver as well as city police and militias were killed by the armed group," the state-affiliated but independent human rights watchdog said.
Four women and children were among the dead, it added.
EHRC said on February 2, gunmen descended on Ano city, which was housing more than 10,000 internally displaced people (IDPs), and "killed house to house."
The gunmen entered an IDP camp and rounded up male residents who could not escape "with some of the deceased bodies burned after their murders," the watchdog said.
EHRC said the killings in Ano targeted ethnic Amharas.
"In the attack business shops, government offices, Commercial Bank of Ethiopia branch and other entities were extensively looted and destroyed," it said, adding that many were injured in the attack.
EHRC said survivors of the violence told them the fighters wore uniforms of the rebel group the Oromo Liberation Army (OLA).
Officials have blamed the OLA for a number of massacres targeting Amharas although the rebels have denied responsibility.
The Oromo, Ethiopia's largest ethnic group, have long complained of marginalization.
The OLA has exploited this growing resentment to swell its ranks in its years-long battle with federal and regional forces.