An object oriented design is independent of the language and you can of course design your system using UML. Some tools like Rhapsody will also allow code generation and round-tripping, we use for some special projects where C++ is not an option. If you want to write your code by hand use some naming convention like Subsystem_Module_Class_Method to name your functions in an object oriented way and use a .c file per class. Using C is not an obstacle against having a clean design.
