a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-guanosine in mRNA + S-adenosyl-L-methionine = a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-(2'-O-methyl-guanosine) in mRNA + S-adenosyl-L-homocysteine + H(+)