Specifically methylates the ribose of guanosine 2251 in 23S rRNA. guanosine(2251) in 23S rRNA + S-adenosyl-L-methionine = 2'-O-methylguanosine(2251) in 23S rRNA + H(+) + S-adenosyl-L-homocysteine Belongs to the class IV-like SAM-binding methyltransferase superfamily. RNA methyltransferase TrmH family. RlmB subfamily.