Oil of Oregano has anti-microbial properties, and that can upset the all-important *balance* of your microbes. It is not that the "bad" microbes are all that bad, it is that there is an improper *balance.*
I've studied herbs for years, and there really is no good reason to take anti-biotic herbs or only a single anti-parasitic herb, and the ways you take them (tincture, inhaled, etc.) also has a lot to do with whether they are effective. The better thing to do is to take probiotics and support the balance of the microbes so that acidopholous and bifidus can do what they do best - keep the more "predatory" kinds of microbes in check themselves.
Also, most people tend to take things like Pau D'arco, Kyolic, oregano, etc., rather than doing an entire regimen of bowel cleansing. To maintain balance, you must have balance in your gut and colon. A filthy colon will contribute to microbe imbalance that no herb can handle.
Psorias is due to having a very congested system - bowels, kidney and liver. What's much more effective than self-medicating yourself with oregano is to do the cleanses as described here, especially the liver flush. The proper order is to do the bowel cleanse, kidney cleanse, then liver. There are forums for each with tons of tips and links to websites to learn more.
You can read up more on this at www.sensiblehealth.com