Does oil of oregano affect effectiveness of antibiotics, specifically Clarithromycin? I have middle ear infection that's not clearing after 5 days.

Not aware of. I am not aware of any such study. Foods in different forms can alter ph of stomach medium, hence always follow directions on the medication in question, in relation to food. It is difficult to isolate Oregano as a cause for poor response. It is likely a drug resistance, before anything else, although Clarithromycin is a broad spectrum product.