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. Required for programmed DNA elimination. 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. Interacts with TWI1. Localizes to the macronucleus. Defects in programmed DNA elimination, and inefficient production of sexual progeny due to reduction in the level and length of piRNAs. Belongs to the methyltransferase superfamily. HEN1 family.