Hi ocd27,

I will explain each photo as I see it and maybe this will help clarify some things.
BTW...thanks for posting the pics!

#1) You have some tartar buildup on both your molar and pre molar which can be irritating to the gum tissue especially if the tartar has migrated to below the gumline.

#2) This photo shows that the molar has a lot of plaque and hard tartar on the mesial surface (area of tooth in between the molar and pre molar). This indicates that you were not able to floss in here properly.
Also, do you see in photo #2 that large red area on the gumline between the molar and pre molar? That is your gum and the onlay was pressing tightly against that area. It appears that this tooth is severely broken and is missing the entire distal (back) portion of the tooth.

WHAT IS THE HISTORY OF THIS PRE MOLAR TOOTH? HOW DID IT BREAK? DID IT HAVE TWO ROOTS AND ONE WAS AMPUTATED?

#3) The onlay is very large in that it is replacing the entire distal portion of this tooth above the gum line.

#4) The discomfort you are feeling when you push on that area of your molar may be due to the accumulation of tartar that might be below the gum line. The fact that you feel the pain in your pre molar when you push here may be referred pain or it may be as you push here you are causing pressure on the pre molar at the same time which is being picked up by the fact that the pre molar is missing it's distal wall and the crown is pressing against the gum tissue.

Bryanna