Methods represent the function calls of Objects. They are defined in OPC 10000-3. Methods are invoked and return after completion, whether successful or unsuccessful. Execution times for Methods may vary, depending on the function they are performing.

The Method Service Set defines the means to invoke Methods. A Method is always a component of an Object. Discovery is provided through the browse and query Services. Clients discover the Methods supported by a Server by browsing for the owning Objects that identify their supported Methods.

Because Methods may control some aspect of plant operations, method invocation may depend on environmental or other conditions. This may be especially true when attempting to re-invoke a Method immediately after it has completed execution. Conditions that are required to invoke the Method may not yet have returned to the state that permits the Method to start again. In addition, some Methods may be capable of supporting concurrent invocations, while others may have a single invocation executing at a given time.