Seems like Audacity does not support VST-plugins that need to be open during processing. I assume that's the case with the Owlsim.
Why do some VST plug-ins not work or display incorrectly?
Audacity will by default display VST effect plug-ins with a full graphical interface where the plug-in supplies this. Any plug-in that requires to be left open while processing audio will not work correctly. VST instrument (VSTi) plug-ins are not currently supported.
If any plug-in displays incorrectly, you can uncheck "Display VST effects in graphical mode" in the Effects Preferences to display a simpler tabular interface for the plug-in (this also affects all other plug-ins).