Incentive stock option agreements (SOS) are contracts between a company and its employees that allow the employee to purchase the company's stock at a pre-determined discounted price. SOS are a type of compensation for employees, and the discounted price is meant to incentivize employees to remain with the company. There are two main types of incentive stock option agreements: non-qualified incentive stock options (SOS) and qualified incentive stock options (SOS). SOS are not subject to tax at the time of grant or vesting, but are taxed when the option is exercised. SOS are subject to taxation at the time the option is exercised, and are subject to special tax regulations.