It's definitely possible to get pregnant 5/6 days before you O, though not as likely as if you'd DTD a day or two before.
This pregnancy was an oops for us. We were using the ol' reliable withdrawal method (lol), and, well... there you go. Hubby and I had kind of decided to stop having kids--he'd even had his initial appointment to get snipped--but I guess God had other plans for us.