It is said that UT99 and TO do not handle a High Mouse DPI and Polling Rate very well. These days a modern "Gaming Mouse" can have maximum DPI levels up to 16000 DPI and a USB Polling Rate up to 2000.
I suggest to use a dpi from 400 up to 800 and a maximum USB Polling Rate of 1000. But this is something you will have test and see what feels best for you.
If you don't know what these are i suggest to Google Search this topic. Most likely your Mouse will not have a higher USB Polling rate of 1000. If you don't know your Mouse DPI Setting try to find a button on your mouse that when you press it, it slows down the speed of the mouse cursor. Try to find the lowest setting and use that for playing TO
Make sure "Enhance pointer precision" is disabled and the "pointer speed" is set to setting 6/11 in the Windows Mouse Settings. This is always recommended no matter what game you play. It is a requirement if you want to use the MarkC Mouse fix combined with playing TO wihout DirectInput enabled.
Most people play with DirectInput enabled. This gives decent mouse aim but gives the known ALT+TAB Crosshair bug, where alt tabbing multiple times can give a white or grey large crosshair on the middle of your screen. Some find this annoying, some use this to make it easier to snipe with the MSG as you now have a crosshair when not using the scope, making it easier to flick to a target before actually zooming.
Most also disable Mouse Smoothing since you don't want your mouse input to be smoothed. But this is a personal choice.
The MarkC Mouse fix is meant to fix the mouse input for older games that use legacy input methods. It is used by players who like to disable DirectInput because they ALT+TAB allot and because it supposedly should give a more 1:1 mouse input then using DirectInput.
Download: MarkC Mouse Fix
To install the MarkC Mouse Fix make sure to check the included readme. Basically you must open the Windows folder of the windows version you are running currently, and select the registry file with the DPI that you currently have in Windows. This is most likely 100% but check your Windows Display settings see to what the "Scale and Layout" Percentage is set too.
At the Main Menu of Tactical Ops go to Tools -> System Console
In the System Console type: Preferences
When you opened the Config Screen, select Rendering -> OpenGL Support -> and scroll down to SwapInterval.
On mouse-sensitivity.com you can convert the sensitivity settings you use in other games to UT/TO settings.