It's good to be cautious, fentanyl is very strong. I would say go with an extremely low starting dose, regardless of your previous opioid use. You can always take more if it's not enough. The initial buccal dose for fentanyl is normally 100 mcg. Then they wait at least 30min and if it wasn't enough take another 100mcg (and so on). And this is for people already tolerant to opioids, it's not normally used for people who are opioid naive. For anesthesia for surgery the starting dose (for buccal tablet) is 5 mcg/kg of body weight. This is how potent fentanyl is. The problem is that you really don't know how it's going to affect you, everyone is so different so it's hard to calculate an equivalent dose for another opioid. We don't really know how fast it's going to be absorbed by chewing or buccally administering a patch either. Even if conversions were accurate and didn't vary so much from person to person, misusing a patch is not going to equate to a normal buccal tablet, so it would be impossible to figure out the equivalent dose to 100mg oxy. Putting a patch in your cheek or chewing it is still likely to release the fentanyl more slowly than a tablet designed for buccal use, but the safest thing you can do is start super low.
The thing you need to do is find out exactly what type of patch you have and how much fentanyl total it contains. As nAON said, a 75mcg/hr patch is going to contain about 12mg of fentanyl. Although I have to disagree that a safe starting dose for an opioid naive person is 375mcg and that if you've done opioids before then you could do 750mcg. I would say an opioid naive person shouldn't be messing with fentanyl at all, and that just because someone has used opioids before does not mean they can handle 750mcg. For a person who normally takes oxy I would start with 1/16th or less of a 75mcg/hr patch. Better safe than sorry.