An oil injector for a vehicle is provided. The oil injector includes a transparent body having a flowing hole formed on a front end of the body. A flexible tube member is connected to the body and corresponds to the flowing hole. A storing space is formed in an internal portion of the body. One end of a piston member is entered into the storing space, and the piston member is movable relative to the body. Therefore, the piston member provides a pushing function in the storing space so that a fluid flows into or out form the body via the tube member, and the transparent body is provided for observing a storing amount of the fluid in the body.