Ever since he stopped taking his morning nap and I have been homeschooling Corbin and Makelle in the morning he has started watching some little alphabet shows that we have to keep him occupied and distracted while we work. We also have the magnetic letters on the fridge the kids play with a lot, foam letters in the bathtub and lots of books about letters that we read, so he has been pretty well exposed to the alphabet letters.
Still, we assumed it was just a coincidence, since 'O' is one of the sounds he can easily make, so to test him we put the O back on the fridge with all the other letters and mixed them up and told him to find the O again. This time he grabbed the O and the zero and handed them to Caleb. We tried a few more times and he found the O or 0 every time.
He was unable to find any other letters we asked him for, but I was still a little amazed. I forget that just because he doesn't talk, doesn't mean he doesn't understand.