An offshore drilling system includes a drilling tower, a tubular string hoisting device with a crown block and a travelling block suspended from the crown block in a multiple fall arrangement. A heave compensation system is adapted to provide heave compensation of the travelling block. The heave compensation system includes a hydraulic sheave compensator. The system further includes a mobile working deck which is movable with respect to the drilling tower within a motion range including a heave compensation motion range. The heave compensation system is further adapted to provide heave compensation of the mobile working deck by a hydraulic deck compensator, which is hydraulically connected via a hydraulic conduit to the hydraulic sheave compensator, such that in operation the deck compensator moves synchronously with the sheave compensator of the heave compensation system.