Had a c-section 8 days ago and was prescribed 5 oxycodone 5mg. i took one today and then googled it and realized it may affect the baby more today because my milk has come in lactmed said his heart could stop or sleep apnea scared?

The dose is at the lower end of common use and a single dose taken just before breastfeeding is unlikely to trigger significant effects. As long as baby is playful & active the side effect is minimal. I would stop using this & rely on non-opioid pain meds or alternatives like heating pads., If you believe baby is too sleepy or inactive have them seen.