To provide an oil ring in which oil consumption can be suppressed from being increased by sticking a lower surface of a lower rail part to a counter surface of a ring groove, and an internal combustion engine.

An oil ring 4 is mounted in a ring groove 3 which is formed on an outer circumferential surface of a piston 2. Furthermore, the oil ring includes: a pair of upper rail part 51 and lower rail part 52 which can be slid, respectively, on a pair of upper counter surface 31 and a lower counter surface 32 which are opposed to each other in the ring groove 3; and a connecting part 53 for connecting the rail parts 51 and 52. In the connecting part 53, an oil return hole 54 is formed throughout a radial direction of the oil ring 4. On a lower surface of the lower rail part 52 of the oil ring 4, a fully circumferential groove 56 is formed which extends over all the circumference of a ring body 5, at a center position in the radial direction of the ring body 5. Moreover, on the lower surface of the lower rail part 52, a connecting groove 57 is formed for connecting an inner peripheral surface of the lower rail part 52 and the fully circumferential groove 56.