The swearing-in ceremony held on Monday at the Government House in Lokoja, the state capital.
Onoja, a former Chief of Staff to Governor Yahaya Bello, was inaugurated shortly after he appeared before members of the Kogi State House of Assembly.
Earlier, the new deputy governor was screened by the lawmakers who then asked him to take a bow, paving the way for his inauguration to replace the embattled former deputy governor of the state, Mr Simon Achuba.
Thereafter, the Chief Judge of the state, Justice Nasir Ajana, administered the oath of office on Mr Onoja who was accompanied by his wife.
The event was attended by Governor Bello, some members of the State House of Assembly, and top government officials in the state.