Anita,

Even though I have a cousin named María de la O I hadn't bothered to investigate the origin of the name until you brought the subject up. It turns out the feast of 18 December was commonly called, even in the liturgical books, "S. Maria de la O", because on that day the clerics in the choir after Vespers used to utter a loud and protracted "O", to express the longing of the universe for the coming of the Redeemer (Tamayo, Mart. Hisp., VI, 485). Therefore Maria de la O Cortes may have been baptized on December 18. The pages with info on María de la O are at http://www.newadvent.org/cathen/05712a.htm and http://www.mariologia.org/reflexionesmarianas347.htm
The only way to be certain that the baptism and marriage of Petronila de San Jose Aguilar is for the same person is if her parents are named the same in both records. They are most likely the same person. Hopefully someone else here can lend more insight.

Armando