The Attorney General is in receipt of your request for an official opinion wherein you ask, in effect, the following question: Where a member of the Oklahoma Teachers' Retirement System has exercised options under prior law to purchase as creditable years of service years in the military service and/or years taught in the school systems of another state, may such member have those non-Oklahoma teaching years purchased credited to his/her years of service for purposes of retirement at age fifty-eight (58) with thirty (30) years of service notwithstanding the provisions of S.B. 490, Thirty-Seventh Legislature, Second Regular Session, 1980, amending 70 Ohio St. 17-101 [70-17-101](24) (1979) which prohibit the utilization of any years other than years of creditable Oklahoma teaching service? In 1978, the Legislature added to the Teachers' Retirement law a provision which, inter alia, permitted members to purchase as creditable years of service up to five years in the military and/or teaching experience in the school systems outside the State of Oklahoma. Laws 1978, c. 238, 11; 70 Ohio St. 17-116.2 [70-17-116.2](C) (1978). For the purposes of our discussion it is not necessary to elaborate on the exact terms upon which the election to purchase could be exercised. It is sufficient to observe that the law provided a member of the System an opportunity to purchase on specified terms for a specific amount up to five (5) additional years of membership to be credited to retirement in either or both of two categories. Nothing in the law as originally enacted precluded the utilization of purchased years of service on any retirement option afforded by the System. The law made no distinction between years earned in the school systems of the State of Oklahoma and years purchased as permitted under the statute. The Board of Trustees of the Oklahoma Teachers' Retirement System interpreted the law as to provide for the application of all years of membership to any retirement option without regard to whether purchased under 70 Ohio St. 17-116 [70-17-116],2(C) or earned in the school systems of this State. See Rules and Procedures, Teachers' Retirement System of Oklahoma, August 1978, Rule II, p. 5-11. In 1979, the First Session of the Thirty-Seventh Legislature, enacted an amendment to 70 Ohio St. 17-101 [70-17-101], Laws 1979, c. 286, 1, which defined for the first time the term "normal retirement age". As enacted, the provisions of 70 Ohio St. 17-101 [70-17-101] (24) originally read: " 'Normal Retirement Age' means age sixty-two (62) for all members except those members with thirty (30) or more years of creditable Oklahoma teaching service for whom the normal retirement age shall be sixty (60)." The last session of the Legislature enacted as part of S.B. 490 an amendment to 70 Ohio St. 17-101 [70-17-101] (24) which reads: " 'Normal Retirement Age' means age sixty-two (62) for all members except those members with thirty (30) or more years of service for whom the normal retirement age shall be fifty-eight (58). For purposes of determining the thirty (30) year service requirement, only creditable Oklahoma teaching service and creditable military service that was immediately preceded by a period of participation in the System and followed within ninety (90) days by renewed participation in the System, shall be considered." The Board of Trustees of the Oklahoma Teachers' Retirement System implemented the 1979 version of 70 Ohio St. 17-101 [70-17-101](24) by promulgating new rules which clearly stated that years of military service or out-of-state teaching years would not be counted for purposes of retirement at age sixty (60) with thirty (30) years service. See Rules and Procedures, Teachers' Retirement System of Oklahoma, Rule II(2)(c) (3)(a)(6), p. 10 11. The 1980 amendment to 70 Ohio St. 17-101 [70-17-101](24) was designed to permit utilization of the purchased military service years for the purposes of retirement at age fifty-eight (58), with the stated limitations. Assuming members purchased years of military service or years of out-of-state service in each of the three years the provisions of 70 Ohio St. 17-116.2 [70-17-116.2](C) and/or 70 Ohio St. 17-101 [70-17-101](24) have been in effect, there are three different situations presented. Persons purchasing years of service after July 1, 1978 (effective date enactment 70 Ohio St. 17-116.2 [70-17-116.2](c)) but before July 1, 1979 (effective date enactment 70 Ohio St. 17-101 [70-17-101](24)) could apply those years, without restriction, to retirement. Persons purchasing years of service after July 1, 1979 (effective date enactment 70 Ohio St. 17-101 [70-17-101](24)) but before July 1, 1980, (effective date S.B. 490 amending 70 Ohio St. 17-101 [70-17-101](24)) could apply those years to any purpose under the Teachers' Retirement law except retirement at age sixty (60) with thirty (30) years service. Persons purchasing years of service after July 1, 1980, continued in the same position as persons who purchased after July 1, 1979, except that some years of military service could be applied to retirement at age fifty-eight (58) with thirty (30) years service. Members who purchased years of service under 70 Ohio St. 17-116.2 [70-17-116.2] (C) prior to July 1, 1979, had a legitimate expectation that the years they bought were legally indistinguishable from any other years of membership for purposes of retirement. Members purchasing years of service after July 1, 1979, could have no similar expectation as they were on notice when they purchased the additional years of the limitations imposed upon the application of those years to retirement at age sixty (60) (or age fifty-eight (58) under the 1980 amendment) with thirty (30) years of service. Statutes are construed as having prospective operations unless the purposes and intention of the Legislature to give them retrospective effect is expressly declared or necessarily implied from the language used. State ex rel. State Bd. of Registration for Professional Engineers and Land Surveyors v. Engineered Coating, Inc., Okl., 542 P.2d 508 (1975). The rule has particular application where substantive or accrued rights of parties are involved. See Jeffcoat v. Highway Contractors, Inc., Okl. App., 508 P.2d 1083, (1972); See also, Board of Trustees of Police Pension R. Sys. v. Kern, Okl., 366 P.2d 415, (1961). Vested rights are property rights which may be acquired by operation of common law, statute or contract. Once property rights have been acquired, they cannot be divested by subsequent law. Moran v. State ex rel. Derryberry, Okl., 534 P.2d 1282 (1975). As applied to the current inquiry, members of the System who purchased years of out-of-state and/or military service under 70 Ohio St. 17-116.2 [70-17-116.2](C) prior to July 1, 1979 acquired certain rights under the law as it existed when they paid valuable consideration for the additional years of creditable service. Since the years purchased under 70 Ohio St. 17-116.2 [70-17-116.2](C) before July 1, 1979, were not restricted as to use, being indistinguishable from earned Oklahoma teaching years of service, the current restrictions of 70 Ohio St. 17-101 [70-17-101](24) cannot operate to preclude application of these years to the thirty (30) years of service necessary to retire at age fifty-eight (58). See 70 Ohio St. 17-105 [70-17-105](1)(a) (1979) as amended. Years purchased under the law as it existed after July 1, 1979, are not creditable to the thirty (30) years of earned service necessary for retirement at age fifty-eight (58) as the law in existence at the time of purchase either prohibited or limited such an application of the purchased years. Therefore, it is the official opinion of the Attorney General that your question be answered as follows: Years of service, whether out-of-state teaching service or military service, which were purchased by a member of the Oklahoma Teachers' Retirement System as authorized by law prior to July 1, 1979, are creditable to the thirty (30) years of service requirement to retirement at age fifty-eight (58), but years of service of the same character if purchased after July 1, 1979, are subject to the limitations imposed by current law. (JOHN F. PERCIVAL) (ksg)