One person was killed and more than 25 injured in the violence in the Kandhamal area on Tuesday, police said.
Christians said it was sparked by Hindus objecting to a performance they were staging to celebrate Christmas.
But a Hindu group said it began when Christians tried to attack a local Hindu leader on Monday, Christmas Eve.
It said a group of people surrounded the vehicle carrying Swami Laxamananda Saraswati as he was on his way to the area. He was taken to hospital but was not seriously hurt.
Now 'under control'
However it started, the violence appeared to culminate in the attacks on churches on Christmas Day.
Christians were chased out of several churches - in many case just mud huts with thatched roofs - before they were set alight, officials said.
One person was reported killed in the violence, but it was unclear whether that was a Hindu or a Christian.
Hundreds of police were deployed following the violence, which had largely died down by Wednesday, a local government official said.
"The situation is tense but under control," said BB Mishra, a state inspector-general of police.
Orissa, which is mainly Hindu and has a tiny Christian minority, has seen violence between the two communities in the past.
The state has a law obliging people to ask for police permission before changing religion - thought to be a measure aimed at Christian missionaries.
Hindus have accused Christian groups of forcing low-caste people to convert.
Christians say they often convert willingly because of their treatment as outcasts.
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