Stock Ownership. Code Section 318(a) applies for purposes of determining stock ownership. Stock underlying a vested option is considered owned by the individual who owns the vested option (and the stock underlying an unvested option is not considered owned by the individual who holds the unvested option). If, however, a vested option is exercisable for stock that is not substantially vested (as defined by Treas. Reg. § 1.83-3(b) and (j)) the stock underlying the option is not treated as owned by the individual who holds the option.