It's a Java applet so there's nothing to install. It "only" checks about 100 programs but they're the key ones.
The OSI page says it takes "5-40 seconds." I saw this all over the place, as high as 4 minutes. Most runs were in the sub-20 second range though.
The first run showed up vulnerabilities in several Adobe products. I'm fanatical about patching Adobe products so that was a surprise.
I finally had to download and save the Flash Player uninstaller. Then closed my browser(s) and ran the uninstaller. When it was done, clicked on the "Show Details" button and looked for "Delete on Reboot..." I found one so I needed to reboot.
After the reboot, I went back to Adobe and installed the current Flash Player.
After that, the OSI ran clean.
Maybe I'll go play with the Secunia Personal Software Inspector (PSI) next.