Just for clarification, this is how I understand the process:

1) You shampoo, this strips the oil
2) Your glands somehow know your scalp/hair is now lacking oil, so they produce more oil to make up for it
3) Repeated shampooing results in your glands continually producing to keep up with the demand for oils - enough oil for your hair to survive shampooing, or rather, more oil than you need if you stop shampooing.