There seem to be a number of caused to such input delays. The symptoms range from input lag or stuttering to long freezes.
One of the first things you should check out is the FAQ's item on tweaking DirectX 8, if you're still running DirectX 8. That's solved this problem for many players.
If you are using NVIDIA Detonator drivers, Tobias Franzén of Sweden says that this may be input lag. This is not caused by a poor framerate. It is caused by frame pre-rendering, which caused all input to get delayed, sometimes resulting in a stuttered effect. This cannot be disabled in the driver settings, even though there may be such a setting there.
BIS says if you're using a GeForce graphics adapter, try unchecking the adapter's ENABLE ALTERNATE DEPTH BUFFERING TECHNIQUE setting. This setting can usually be modified through the adapter's PROPERTIES window, under ADVANCED and then wherever the adapter's Direct3D settings are displayed.
This problem is not specific to OFP. The problem can be resolved by using the RivaTuner utility program (available from Guru3D), for Nvidia and Radeon graphics adapters, to minimize pre-rendering. Set the pre-render limit to "1" (default is 3). Setting the pre-render limit to "0" may result in lockups in DirectX games and in errors when running DXDiag. Also try turning off the D3D vsync option.
Some people have resolved this problem by switch off anti-aliasing. Other people said the problem disappeared when they shut down all other Windows applications (other than SysTray and Explorer - leave those two alone).
If you've got a joystick or game pad device defined to Windows, take a look at this FAQ item. Similar symptoms have also been reported as being caused by scanners and other peripherals.
OFP player Kendal McGuire adds that if you've got Windows XP, playing OFP in compatibility mode for earlier Windows versions can also slow your gameplay down.
Another suggestion I received was to open up your mouse drivers from Windows Control Panel, click on the Pointers tab and make sure that ENABLE POINTER SHADOW is unchecked.
OFP player Arne resolved his lag problem by decreasing his screen resolution size in the OFP Preferences program. Of course, for those of you with notebook PCs that have fixed resolution screens, this is not a recommended solution.
One other thing that personally helped me was tinkering with the mouse SPEED and ACCELERATION setting. Once again, this can be accessed through the mouse driver's Properties interface. For myself, I set SPEED midway and ACCELERATION to HIGH. I found that maximizing the SPEED setting caused my in-game gun site cursor to jump whenever I tried aiming accurately at a target with my weapons. Of course, your experience may vary.