The theory of Observational Relativity (OR), as a new theory in human being's physics, has revealed the essence of the relativistic effects of Einstein relativity theory, and moreover, has generalized and unified Newton's classical mechanics and Einstein's relativity theory in the same theoretical system under the same axiom system. However, the original intention of OR research is not to establish the theory of OR, but to give photons some rest mass. According to the mass-speed relation of Einstein special relativity, an object of matter moving in inertial spacetime exhibit two distinct masses: the rest mass; the moving mass. However, as an object of matter reaches the speed of light, either its moving mass becomes infinitely large or its rest mass becomes infinitely small. Einstein chose to set the rest mass of photons to zero. It is puzzling that Einstein's moving mass depends on observation: the same material object has different moving masses relative to different observers. Therefore, people subconsciously believe that Einstein's moving mass is not objective and real, and that only the rest mass is the objectively real mass with real inertial effects and real gravitational effects. So, if photons had no rest mass then they would have no mass. According to the materialist view of nature, the natural world is a world of matter, and all matter or material particles, including photons, have the intrinsic and objectively real mass. Naturally, the objectively real mass must be independent of observation. OR serial report 1 has elucidate the theoretical validity and empirical basis of OR. Now, OR serial report 2 will report on the exploration for the objectively real mass of photons by the theory of OR. The theory of Inertial OR (IOR) proves that photons possess the rest mass; the theory of Gravitational OR (GOR) predicts that the theoretical value of the rest mass of a photon with the frequency f is mo=hf/c2. Thus, according to the theory of OR: all matter or material particles possess the objectively real rest mass; a material object, whether it is a massive star or a tiny photon, must possess the rest mass of its own if it exists objectively; otherwise, it does not exist.