Opus doesn't have if...then...else (or similar) built in but when you need it you can do it in a couple of ways:
[ol][li]Call into a script. This is my preference. Create a bit of VBScript and call it from Opus, passing the relevant arguments. If you need to call any Opus commands from the script you can do so with "dopusrt.exe /cmd " or you can handle everything in the script itself. There's an example of this here.
[/li]
[li]Create an MS-DOS Batch type of button in Opus and have the if...then...else as part of the button itself. This saves you from creating an external script file but, IMO, is worse on balance since MS-DOS batch files are so arcane, ugly and limited in what they can do compared to a real scripting language. I'm not sure but you may still need to use dopusrt.exe to run Opus commands in such a batch-button else the Opus commands may be executed regardless of any conditional logic.[/li][/ol]