The incident took place on a major highway connecting the southern province of Kandahar with the capital city of Kabul.
Ghulam Jilani Farahi, deputy police chief of Zabul province where the accident occurred, said authorities could identify only six bodies and the rest were totally burnt.
Farahi said several women and children were among the victims in the bus that was carrying more than 60 people.
The driver of the oil tanker and a co-passenger died immediately after the truck burst into flames during the early morning hours on Sunday.
The Kabul-Kandahar highway passes through areas prone to militancy and many drivers are known to drive at top speeds in hopes of avoiding insurgent activity.
Afghanistan has some of the world's most dangerous roads, often in dilapidated condition, and traffic rules are seldom enforced.
