Olive Oil is one of the few oils that you can use at 100% in soap making. This means that you can make a soap that consists of only olive oil, water, and lye. This type of soap is called Castile Soap, and is a very mild and nourishing bar. The one downside of castile soap is that it takes a very long time for the soap to fully cure. Soaps made with a combination of oils typically take 4-6 weeks to cure, although some may benefit from longer cure times. Castile soap requires a much longer cure time, up to a full year!
Since most of us don't want to wait a year to be able to use a batch of soap, we generally use a mixture of oils. However, olive oil is still used in high percentages in many recipes. It helps to add a lovely dense and creamy lather to your soap, and can help trace stay relatively slow when used in higher percentages, allowing the soap maker to create various swirls from colored soap batters. Castile soap is notoriously slow to trace due to being 100% olive oil.
There are a lot of different kinds of olive oil available. We've probably all used extra virgin olive oil for cooking, but there are other varieties as well. These include pomace, virgin, and pure (also sometimes called Grade A). Any of these can be used in soap making, but the cost of some of these can be prohibitive. I typically use pure olive oil in my soaps, as it is both readily available and the most economical. I also like the pure olive oil because I have found that it tends to be lighter in color than pomace or extra virgin. This means that my oils aren't as yellow or greenish, and it is easier for me to achieve the colors I want in the finished soap.
Olive oil has been used for centuries as a beauty product. It can also be used on the skin as a type of lotion. It contains several antioxidants and absorbs readily into the skin.