A data model named OIM is proposed in this paper as the common data model for integration of heterogeneous data sources. Based on rooted connected directed graph both cyclic and acyclic, OIM can describe the relationship of a complex object and its component objects, as well as the links between HTML files in World-Wide-Web naturally. As the metadata is associated with each object, OIM is especially suitable for describing objects without explicit predictable data schemata. As the formal foundation for query decomposition and optimization, a new algebra called OIM algebra, which includes six operations, i.e., object union, difference, select, project, paste and cut, is proposed. In comparison with relational algebra, OIM algebra is more flexible and powerful.