OSM stands for Object-oriented Systems Modeling. The focus of the OSM research group is the development of a method for exactly specifying the behavior of a system of "objects." This method is called OSA, or Object-oriented Systems Analysis. OSA can be used to represent objects' structures, behaviors, and interactions.
An OSA Tutorial is available.
The Composer program helps you draw diagrams using OSA.
For example, the following is a simplified model of a car:
Figure 1. Car Object Relationship Model
The black triangle indicates an aggregation relationship between Car and Wheels, Frame, and Engine. In other words, this diagram says that a Car is is made of of Wheels, a Frame, and an Engine.
This type of diagram is called an Object Relationship Model (ORM). The purpose of ORMs is to show relationships between types of objects (eg. between a Car and a Frame).
Another example is the following model of the behavior of the lock on a car door:
Figure 2. Lock State Net
The two ovals with green text represent the states the door lock can be in. The door can either be locked or unlocked. When the door is locked, and the lock receives a message from the out side to lock the car door, then the door lock locks the car door and enters the locked state. The reverse happens to unlock the door.
This type of diagram is called an Object Behavior Model, or State Net. A state net shows how an object behaves. The jagged lines with the black labels indicate an interaction with another object.