Oyo state police spokesman Adekunle Ajisebutu said today that 11 others were injured in the crash.
Police said the two buses were trying to overtake other vehicles on the Lagos-Ibadan Expressway and ended up striking each other.
Road accidents are common in Nigeria, where more than 5,000 people died in crashes last year, according to national statistics.
Many accidents are caused by reckless driving, while others are attributed to poorly maintained roads and vehicles.