Centre of suspension:

Centre of suspension of a compound pendulum is a point where the vertical plane in which the compound pendulum oscillates, meets the horizontal axis of rotation of that compound pendulum.
In Fig. 1, the point O is called the centre of suspension, where the horizontal axis of rotation of the compound pendulum meets the vertical plane of oscillation of the that compound pendulum.

Centre of oscillation:

For a given centre of suspension of a compound pendulum, there is a point, where the total mass of that compound pendulum may be considered to be concentrated without causing any change in the time period of oscillation of the compound pendulum.
In Fig. 1, O’ is called the centre of oscillation for the given centre of suspension O of the compound pendulum..

The distance L, between the centre of oscillation (O’) and centre of suspension (O), is equal to the length of the equivalent simple pendulum.