OIC chief Abd al-Wahid Bilqaziz branded Johnson's beheading a barbaric act, in a statement issued at OIC headquarters in the Saudi Red Sea city of Jidda.
"Islam recommends the protection of foreigners ... and prohibits killing innocent people," he said.
Bilqaziz said the killing "distorts the reputation and image of Islam for foreigners, which harms Muslims around the world."
He also underlined the organisation's solidarity with Riyadh in "its campaign to destroy those seeking 'sedition' in Saudi Arabia."
Arab world comment
In Cairo, Arab League Secretary General Amr Musa reaffirmed "the need to put an end to the terrorist operations which have struck Saudi Arabia".
Bahrain's King Hamad described the killing as a "criminal act perpetrated by a group of saboteurs".
A Qatari foreign ministry spokesman said Johnson's murder was "against all precepts of Islam, all moral values and all humanitarian principles".
In another foreign ministry statement, Oman denounced the killing as an "unjustifiable act against the tolerance of Islam".
Kuwait's cabinet expressed its "strong condemnation and indignation", describing the murder as a "brutal and inhumane act rejected by Islam and the human conscience".
72 hour warning
On Friday, a group calling itself Al-Qaida in the Arabian Peninsula posted photographs on websites showing the beheading of Johnson, an American aeronautics engineer.
He was kidnapped on 12 June. It had earlier said he would be killed in 72 hours if prisoners held in Saudi jails were not released.
Shortly after the beheading, Saudi police shot dead the group's chief – Abd al-Aziz al-Muqrin and three of his associates.