Abstract: An O-ring assembly tool for locating an O-ring within a groove formed in the end surface of a fitting includes a plunger having a cylindrical body with an end wall. A locator hub with a cylindrical body closely receives the cylindrical body of the plunger. A compression spring biases the plunger rearwardly within the locator hub, while a retaining pin allows axial forward movement of the plunger, but prevents relative rotation with respect to the locator hub. The locator hub includes an enlarged receiving end which axially and radially locates the assembly tool with respect to an end fitting. The cylindrical body of the plunger includes a flat annular engagement surface and when the plunger is pushed forwardly, the engagement surface pushes an O-ring forwardly into the groove on the end face of the fitting. The locator hub has a tapered inner surface at its forward end which radially-inwardly compresses the O-ring to fit cleanly within the groove.