Data Flow Daigram and Object Model Daigram

What is difference between Data Flow Daigram and Object Model Daigram ? Are both related to Design phase ? Which use in which case ?

In DFD, we try to model the flow of data or information in the system. Developing DFD helps us to understand and analyze the proposed system. This is first step of design phase but since the phases are overlapping in development so you can also consider it in requirement analysis. DFDs are one of the few ways to understanding system being developed.
 In Object Model Diagram, the purpose is to describe the relationship between different objects of the system. This diagram mainly focuses on some particular set of objects and attributes, and the links between different objects. Object Model Diagram is part of design.