What I am about to state only applies to California's poison oak. Other plants may be different.
Tecnu or something like that can go on your skin. Long clothing covers the skin as well. If you still get the rash, I can guess about how that happened. It happened when you removed the contaminated clothing, got the oils on your fingers, and then migrated it to other skin surfaces. When you remove contaminated clothing, wash that clothing immediately before it can spread the oils to anything else. It is particularly easy to pick up the oils if your skin is sweaty.
Poison oak plant looks green and fresh in the springtime. The oils are on the leaves. Then it turns green and red during summertime, and the oils are still on the leaves. In the autumn, it turns brown and then the leaves drop, exposing woody stems. The oils are still present, but you won't get as many of the oils onto your skin. (However, the last time I got a rash by the coast, it was from walking past a thicket of woody poison oak bushes without leaves.)
The plant can grow as a bush, or it can send a vine up along a tree trunk. I've seen poison oak vines 60 feet up in a tree.