Answer:
Technically you can take them together, however, this is extremely dangerous. Although injury's or death are not common on low dosages of Methadone and use of Oxycontin, i would not recommend it. Especially if you took10mg or more, i would wait at least 24 hours. It should still continue to flow a good amount throughout your bloodstream for at least 10hours.
Depending on the dosage of Methadone you have taken, depends on how long it will not overpower the Oxycontin. Methadone is extremely stronger than Oxycontin, so you may not feel it if you decide not to wait. Some people consider it a waste of an OC.
Not matter what, be care. There are a few stupid doctors that will Rx Oxycontin and Methadone at the same time because they consider it an 'additive' drug. No, they are considered 'Legal Drug Dealers.'
Good luck,
-H