What is the phase difference between the oscillation of the electric field and the oscillation of the magnetic field?

Answer: Zero

Explanation:

In electromagnetic wave, the magnetic and electric field are perpendicular to each other as well as perpendicular to the propagation wave. Both are always in a phase.

When the electric field is minimum, the magnetic field is also minimum and when the magnetic field is maximum, the electric field is also maximum. They are both in the same phase. Therefore, the phase difference between the oscillation of the electric field and that of the magnetic field is zero.