XML (eXtensible Markup Language) has drawn significant attention as a standard for representation and exchange of data on the Web . But existing semi-structured data models, such as OEM, can not support XML naturally and fully. A new object oriented data model named XOM is proposed in this paper as the common data model for the Web data source. Based on rooted connected directed graph both cyclic and acyclic, whose nodes are labeled and edges are ranked, XOM can describe the aggregation relationship as well as the general reference relationship (linkage) between XML elements. Through ranked closure operation, both tree-oriented and graph-oriented models can be unified in XOM. As the formal foundation for query processing and optimization for Web data, a new algebra called XOM algebra, which includes six operations, i.e., object union, difference, select, project, paste and cut, is also proposed. In comparison with relational algebra, XOM algebra is more flexible and powerful.