Methyltransferase that adds a 2'-O-methyl group at the 3'-end of selected small RNAs. Mediates 2'-O-methylation of piRNAs, single-stranded siRNAs and long hairpin RNA genes (hpRNAs). Methylation protects the 3'-end of small RNAs from tailing and trimming and could constitute a recognition signal for appropriate argonaute machineries. S-adenosyl-L-methionine + small RNA 3'-end nucleotide = H(+) + S-adenosyl-L-homocysteine + small RNA 3'-end 2'-O-methylnucleotide Binds 1 Mg(2+) ion per subunit. Belongs to the methyltransferase superfamily. HEN1 family.