Oil paints are sensitive to air, moisture, high temperatures, and direct sunlight. When they are exposed to air and moisture, they tend to harden up and become brittle. At the same time, if the paints are kept under direct sunlight, the colors will start to fade with time.
However, you can easily protect the oil colors by storing them in a dark, dry, and cold place inside an airtight container. Modern oil paints mostly come in airtight and resealable metal tubes that make storing the colors much easier.