In Tactical Ops you can't change the FOV settings the same way as in UT99, there is no way to change the horizontal FOV. This means no matter the resolution you play on, 16:9 or 4:3 the area you can see from left to right is always identical.
The only thing you can "change" is the vertical FOV, the area you see from top to bottom is impacted by the resolution used. It's done in a way where the more "square" your resolution is the more you can see of the game world.
Unlike as in modern games, when you play TO in 16:9 widescreen you gain 0 extra visibility, all that happens is that the top and bottom of the game world of are chopped off to fit a "square" image unto a widescreen monitor.
They are sorted from highest Vertical FOV (see most top to bottom) to lowest:
5:4 - 1280x1024 etc.
4:3 - 1024x768, 1152x864, 1280x960, 1440x1080 etc.
16:10 - 1680x1050, 1920x1200 etc.
16:9 - 1280x720, 1360x768, 1920x1080, 2560x1440 etc.
Things To Notice
Since the Vertical FOV is lower on 16:9 than on 4:3 there can be situations where you won't see an enemy that is above or below.