Look, the "downer" did the research for you, and i dont even care.
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http://www.ayahuasca.com/?p=10
As to oxycodone (and codeine), a recent review article (Gillman 2005) suggests that it does not provoke serotonin toxicity with MAOIs because it lacks serotonin reuptake inhibitory property (at the difference of DXM and methadone for instance). Yet, if one wants to err on the side of total extra absolute safety, simple maths based on the longest known terminal half-life of oxycodone (w/ slow release tablets) give a delay of 18 hours to wait before drinking the tea.
Reference
Gillman, P.K. (2005). "Monoamine oxidase inhibitors, opioid analgesics and serotonin toxicity". British Journal of Anaesthesia, 95/4: 434-441 (abstract available here).