Final answer:
The O horizon has more organic matter compared to the A horizon, which is a mixture of organic and inorganic substances and marks the beginning of true mineral soil.
Step-by-step explanation:
The primary difference between the O and A horizons is that the O horizon usually has more organic matter. The O horizon contains freshly decomposing organic matter, known as humus, at its surface, with decomposed vegetation at its base. This layer enriches the soil with nutrients and enhances soil moisture retention. On the other hand, the A horizon, which is considered topsoil, consists of a mixture of organic material with the inorganic products of weathering and is the beginning of true mineral soil with a typically dark color due to the presence of organic matter. In contrast, the B horizon, or subsoil, accumulates mostly fine material that has moved downward from the A horizon, and the C horizon contains the parent material of the soil.