What are the main characteristics of the O horizon?
Its called "O" for organic horizon and therefore consists mostly of plant material and the dead stuff that is in various stages of decay..
The "O" horizon is near the top of soil horizons and also contains a thriving ecosystem of decomposing bacteria. worms and other critters who break down the organic material. Over time, the O horizon becomes part of the A horizon.
Not all soils have an O horizon (e.g. grasslands don't), and most form in forests or places where there is abundant plant material.