@turquoise-- Yes, I use tea tree oil for psoriasis regularly. I buy the essential oil and dilute it with a carrier oil or add it to my bath. I think that this is better because I can control how much tea tree oil I use.
Tea tree oil can be very irritating. I cannot use it undiluted because it burns my skin. Most products with tea tree oil in it don't clarify on the packaging how much tea tree oil they contain. So I'm scared to try those.
When I buy the essential oil, I use a dropper and add only a few drops to sweet almond oil before I apply it on my skin. Once in a while, I add a few drops to my bath water as well. I think that this is more effective and the oil lasts longer.