what is Oh blood group? which antigens antibodies are present in it? how can we distinguish it from O blood group?

It is a rare blood group also known as bombay phenotype because it is first observed in Bombay. In this blood group, the antigen present on the surface of RBC's  is neither A or B, rather it has a third antigen 'H'. It is different from O blood group because in O blood group no antigen is present whereas in Oh blood group H antigen is present in (hh) homozygous condition.