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 and adenylation activities and subsequent degradation. Stabilization of piRNAs is essential for oocyte development. 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. Gonad-specific. Expression in gonads starts around 3 weeks of age, which corresponds to the start of sex determination. Expression remains present in the adult gonads, both in males and females, although expression is weaker in testis. Defects in germ cell development due to reduced piRNAs levels. Belongs to the methyltransferase superfamily. HEN1 family.