I have created one custom object A. I related this object with Opportunity. I created one custom button in custom object. When I click the button I need to get the Optortunity Row Id and pass it to external system.
If I define the OpportunityVO itself it throwing error.
You'll have to use the internal "bindings" to get this value from the session context. As a general recommendation, look at the properties of other pages where this is included (incl. hidden) and see if you can derive the groovy expression from that. You can see an example in this video.