I got a HUGE mouth sore last year about 2-3 weeks after starting OP (I think it was after I began adding a little oregano oil, and my theory was that it was a dormant HSV virus coming up and out). It took about 10-12 days to go away and it was completely miserable the whole time and my heart goes out to you! Not fun.
Last spring several people had mouth sores, perhaps from detoxing Rx drugs or other non-microbial toxins, so unless you are someone who gets canker/mouth sores regularly it may just be toxins coming out faster than your body can process them. I OP'ed throughout the sore episode and it did not get worse. I also took L-Lysine and ended up taking narcotic pain meds it was so bad.
About the best I can say is that it will go away...I did not find any topical stuff helpful because it would not stay on. I would be willing to bet that cayenne would stop the pain, but I would also be willing to bet it would hurt like the dickens while it was kicking in, which might be more than you want to bear (I used straight cayenne several times up my nose this summer for migraine relief and it worked really well after that first 20-30 minutes of fiery beyond-belief pain...clove oil is supposed to be good, too.) Nowadays with a mouth sore I would probably swish with a little Lugol's and raw milk.