Based on the object—oriented database system FOOD which has been developed by Fudan University，a deductive query language O—Datalog is presented in this paper．O—Datalog language can easily express various requirements to query and deduce from the object—oriented database．It can be translated to a Datalog—like form，SO the O—Datalog programs can be evaluated using the efficient algorithms for evaluating Datalog programs．It'S easier to implement the language than to implement those based on non—Horn logics．O—Datalog is firmly an extension of Datalog in form．The syntax and the semantics of the O—Datalog language are discussed in this paper．