Oil paint is made of plant-based oils and pigment, usually linseed, safflower, or walnut oil.
Painting in oil can be as simple or complex as an artist wants to make it, whether it is using additives in the paint or technique. Oils layer beautifully, but a painting can also be finished in one sitting. This is called alla prima painting.
Soft Pastel is essentially dry paint. They are made of pigment and binders to hold the sticks together. Pastels come in varying degrees of softness, depending on the brand. Different papers will also give different effects.
Gouache is opaque watercolor. I started painting in gouache because there were places I wanted to work on-location that I cannot take my oils or pastels. The Museum of Natural History is one of my favorite places to paint and gives me access to subject matter I could not paint from life otherwise. My palette is the same as what I use for oils, and I use a cigar box mounted on a small tripod.
Plein air is possibly the most difficult practice as far as painting goes. This is because you are literally in your painting. There are endless compositions to choose, and you have to deal with the constantly changing light. It can also be physically unpleasant due to temperature, wind, bugs, and whatever else the environment throws at you..
Despite the hardships of plein air, it is special and deeply enjoyable. There is a story behind all my plein air paintings, and I have had many unique adventures on my plein air outings.