In the system32 directory I saw an .OCX file with a corresponding .OCA file.
I had thought .OCA files are used only by Visual Basic. Are they therefore unnecessary for program execution and could be removed?
If they are unnecessary, why would there be an .OCA in the system32 folder in the first place?
.OCA files serve as a cache of the extended type library information for its corresponding .OCX file.
If you delete an .OCA file for a control VB recognizes and uses, VB will recreate the .OCA file when you load a project requiring the control. The recreation process takes a little time but comes with no penalty otherwise.
Source: KB article - What is an OCA file?
So, don't worry about including them when you deploy your application.