An optimistic scheme, called ODL, which uses dummy locks to test the validity of a transaction for concurrency control in distributed database systems, is suggested. The dummy locks are long-term locks; however, they do not conflict with any other lock. By the use of long-term dummy locks, the need for the information about the write sets of validated transactions is eliminated and during the validation test only the related sites are checked. Also, the transactions to be aborted are immediately recognized before the validation test, and therefore, the costs of restarts are reduced. Furthermore, usual read and write locks are used as short-term locks during the validation test. The use of short-term locks in the optimistic approach eliminates the need for the system-wide critical section and results in a distributed and parallel validation test.