Ethanolamine phosphate transferase involved in glycosylphosphatidylinositol-anchor biosynthesis. Transfers ethanolamine phosphate to the GPI third mannose which links the GPI-anchor to the C-terminus of the proteins by an amide bond. Glycolipid biosynthesis; glycosylphosphatidylinositol-anchor biosynthesis. Forms a complex with PIGF. PIGF is required to stabilize PIGO. Belongs to the PIGG/PIGN/PIGO family. PIGO subfamily.