Mo. Rev. Stat. § 104.1039
If a retiree is employed as an employee by a department, the retiree shall not receive an annuity payment for any calendar month in which the retiree is so employed except, notwithstanding the provisions of section 105.684 to the contrary, those retirees serving as a member of the general assembly or as a statewide elected official under section 104.1084. While reemployed the retiree shall be considered to be a new employee with no previous credited service and must accrue credited service continuously for at least one year in order to receive any additional annuity. Such retiree shall receive an additional annuity in addition to the original annuity, calculated based only on the credited service and the pay earned by such retiree during reemployment and paid in accordance with the annuity option originally elected; provided such retiree who ceases to receive an annuity pursuant to this section shall not receive such additional annuity if such retiree is employed by a department in a position that is covered by a state-sponsored defined benefit retirement plan not created pursuant to this chapter. The original annuity and any additional annuity shall be paid commencing as of the end of the first month after the month during which the retiree's reemployment terminates. Cost-of-living adjustments paid under section 104.1045 shall not accrue while a retiree is employed as described in this section except, notwithstanding the provisions of section 105.684 to the contrary, those retirees serving as a member of the general assembly or as a statewide elected official under section 104.1084. Any future cost-of-living adjustments paid after the retiree terminates such employment will be paid in the same month as the retiree's original annual benefit increases were paid.
§ 104.1039, RSMo