Methyltransferase that adds a 2'-O-methyl group at the 3'-end of piRNAs, a class of 24 to 30 nucleotide RNAs that are generated by a Dicer-independent mechanism and are primarily derived from transposons and other repeated sequence elements. This probably protects the 3'-end of piRNAs from uridylation activity and subsequent degradation. Stabilization of piRNAs is essential for gametogenesis. 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. Component of the meiotic nuage, also named P granule, a germ-cell-specific organelle required to repress transposon activity during meiosis. Belongs to the methyltransferase superfamily. HEN1 family.