If I use Tor browser, can websites know my OS? For example, if I use different OS than Windows and websites know it, I will be different and it would reduce my anonymity.
No, websites will always think that you are using Firefox on Windows 7 even if you are using Mac, Linux, or some other version of Windows.
If you use some other browser with Tor other than Tor Browser, then that browser's identify will be given unless you make the effort to hide it.
