Catalyzes the 2'-O methylation of guanosine at position 18 in tRNA. guanosine(18) in tRNA + S-adenosyl-L-methionine = 2'-O-methylguanosine(18) in tRNA + H(+) + S-adenosyl-L-homocysteine Stimulated by magnesium ions and spermine. Inhibited by S-adenosyl-homocysteine. Optimum pH is 7.2-7.5. Monomer. Belongs to the class IV-like SAM-binding methyltransferase superfamily. RNA methyltransferase TrmH family.