I agree that there are not many scientific reports on the efficacy of companion planting and plant extracts used as pesticides, but I found at least one on oregano oil: Oregano Oil Works As Well As Synthetic Insecticides To Tackle Common Beetle Pest (
http://www.sciencedaily.com/releases/2008/05/080522072339.htm), which was published in Journal of the Science of Food and Agriculture in 2008.
The listed benefits of oregano seems seem to be nearly endless:
'Analgesic, anthelminthic, antibacterial, antifungal, antimicrobial, antiseptic, antispasmodic, carminative, cholagogue, diuretic, emmenagogue, expectorant, fungicidal, tonic.
It’s Benefits: Acne, bronchitis, colds, congestion, flu, infections, insect bites and stings, muscular aches and pains. Well diluted, oregano oil can be added to ointments and balms for skin abrasions.' (
http://www.pestreport.info/bacteria/antimicrobial-cytotoxic-activities-oregano-essential-oils/), but no further references to scientific reports verifying this.