Who gave octavian the title of Augustus?

Octavian gave himself the name of Augustus.

Octavian's original name was Gaius Julius Caesar Octavianus and his mother was the sister of Julius Caesar.

In 27 BCE he became Gaius Julius Caesar Augustus in recognition of his services to the state as the first of the Roman Emperors. All succeeding Caesars (Emperors of Rome) called themselves Augustus after that.