How long do i have to wait to use a Fentanyl 100 mcg patch after taking a oxy 80 mg?

Everyone's different, but in general it takes several hours (especially if you're already opiate tolerant by using OxyContin) for Fentanyl to achieve a dosage equivalent to what you're taking. You want to adjust your patch attachment time so that when your OxyContin dose is going down, the patch dose is ramping up.

Your doctor should have already told you this; keep in mind that initial patch doses are skewed toward the low end of the dosage chart for safety, so your initial dose might not cut it. Make sure you've got enough breakthrough meds available until you can get your patch dose correct.