It's probably normal, although I couldn't say for sure without seeing what the events are. But computers do a lot of things very quickly, and there's a constant stream of activity from lots of processes even when you aren't doing anything with the computer. 
Automatic size calculation will cause a lot of disk access if you point Opus at a folder with lots of things below it. Windows and NTFS are quite inefficient when it comes to reading directory listings.
See Process Monitor instructions for guidance.