Answer:
The two are very close. Oxycodone differs in that is has a hydroxyl group at Carbon 14 (hydrogen/oxygen rather than just hydrogen), hence the name "Oxy" in Oxycodone.
Of the 2, Oxycodone is the more potent, being 15-20 times stronger than Codeine; Hydrocodone, by comparison, is only 6 times as strong.