The invention relates to an air compressor mountable in an annular tire cavity of a pneumatic tire. The compressor comprises an air inlet, an air outlet, and a body in which are formed a blind hole having a sidewall and a compression chamber in fluid communication with the air inlet and the air outlet. Further, the tire comprises a compression piston configured for reciprocating in the compression chamber for providing compressed air to the tire cavity through the air outlet, and a hydraulic fluid passage for providing hydraulic fluid into the blind hole, as well as a guiding piston configured for reciprocating in the blind hole in sealed contact with the sidewall and for guiding a reciprocating movement of the body relative to the compression piston. The invention further relates to a tire assembly comprising a tire and the compressor.