Depends on what you mean by OO model. One of the most important things about Smalltalk is 'thisContext'.

The various links referenced from:

http://stackoverflow.com/questions/6347599/how-does-smalltal...

Should give you a good idea.

Basically, the current call stack is available. This makes it trivial to write a debugger, restart processing from a previous point in execution (debugger driven development as it is sometimes called makes the TDD process make so much sense when you do it in Smalltalk).