First person singular.
Old Spanish -o or -ó. The explanation for the unexpected development from -o to -oy is unclear. The most probable theories are: contamination from yo (e.g. so yo > soy yo; do yo > doy yo) , Müller (1963); and (2) contamination from the now-extinct word y "there," which is known to be the origin of <y> in hay "there is" (see discussion and sources in Lloyd 1987).