Hey wingchun, does the original article you posted describe how they obtained their extract? That would be the obvious route to take, if hoping to replicate some of their results.
Oregano contains essential oils (high in carvacrol, thymol and p-cymene, with other trace constituents), and it is likely the volatile oil fraction responsible for MAOI activity. It's usually obtained by steam distillation of the plant material, more rarely by solvent extraction (if you find this for sale it would be called an absolute). The steam distilled essential oil is easy to buy, or if you go to a health food store you will probably find it there sold as "oil of oregano" which is usually diluted into a carrier oil of some kind.
Be cautious if working with the pure essential oil! It is so spicy and concentrated it may cause burns to your mouth. For therapeutic use, it is generally diluted to a 1-5% concentration. Also be warned, it is a very powerful antimicrobial (sold at health food stores to treat colds), and if you use it regularly it can strip some of the beneficial gut flora in the digestive tract.
I question whether this is a viable MAOI to potentiate tryptamines, or whether it is in the same category as yerba mate, yohimbe, kava kava etc, which are all known to have some MAOI activity but do not seem appropriately strong to make DMT orally active.