At least 36 people were killed when unidentified attackers struck three villages in Oromiya's Shirka district on November 24 and 27, residents said.
Hailu Adugna, the Oromiya region's spokesman, blamed the attacks on the OLA, an outlawed splinter group of a formerly proscribed opposition party that returned from exile in 2018.
"The terror group is committing brutal acts on civilians because it is now very anxious," Hailu told the state news agency.
Property was also destroyed in the attacks, he added.
There was no immediate comment from the OLA. Federal government spokesman Legesse Tulu did not respond to a request for comment.
The government and its Oromiya regional counterparts have mobilized security teams to push out the rebels from areas they control, Hailu said.
The latest attacks came days after a second round of unsuccessful talks were held in Tanzania between the government and the OLA to resolve the decades-old conflict.
Seventeen people were killed on November 24 when attackers raided one village and 11 were killed on the same day in another village, residents of the area said.
One farmer lost seven members of his family, including his wife and son. Two of his other children suffered injuries and were being treated in hospital, the farmer said.
"They targeted Orthodox Christians," he said.
Another eight people, including babies, were killed in an attack on a third village in the area, the residents, who included a church leader, said.
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