"a" and "o", considering there's no "a" in the word?
Well, since those two letters are right next to each other on a standard keyboard, it's easy to see how you made that mistake. 
Lets see I can dumb this down for ya....
I spelled POST P-O-A-S-T
By your own convienient pic of the keyboard, we see the A and the S located right next to each other...
First...I typed the P, as it was the first letter in the word.
Then...I hit the O
Then....I inadvertantly hit the A along with the S, I was intending to press...followed up by the all important T to complete the word.
Perhaps YOUR keyboard doesn't have the A and the S located next to each other, in which case I apologize.