ok well I am going to assume that since you said OC you ment oxycodones extended release version which provides a controlled release of medication for 12 hours. Since you said that you take them orally and you said nothing about crushing or chewing them I am also going to assume that you just swallow them whole as directed. If an am incorrect please correct me. Now if these facts are true and you normally take two tablets then per hour your body is getting 1.6 mgs per tablet. Since you take two tablets your body is getting 3.33 mgs per hour. If this is so then a dose of 5 mgs of methadone released all at once would be a high dose and might make you sick as you said. then again you did say that when you take two together you feel good for about an hour~ which doesn't really make sense because peak plasma levels aren't reached for three hours and then they are maintained for longer than oxycodone IR tablets because of the release mechanism. Also I don't know if this is really in the correct forum.
That's just my guess. You should check out the mega thread on "swim" use.