Yes there is! One way is to use this small OFP mission save utility for campaign missions. (NOTE: This utility does not work with OFP: Resistance version 1.75 and above). To install, just extract the contents of the ZIP file into the directory of your choice.
To use, run the utility program before you run OFP and leave it in the
background. Start OFP and play as usual. When you save a mission for the first time, just do
it normally as you always have until now.
When you next want to save during mission play, press your keyboard's PAUSE/BREAK key. This will cause the save utility running in the background to rename the existing save file. This will allow you to save your mission again from the mission Pause menu.
If you want to restore one of your previous mission save files, press the ALT+TAB keys to go back to Windows and bring up the save utility's menu. Select the RESTORE option from the save utility's menu and then select the save file to restore. The utility's saved file names contain a date and time stamp. After restoring a saved mission file, go back to OFP, press your keyboard's ESC key to display the mission Pause menu and click on LOAD to use the restored mission save file.
OFP player Dan "Haddock" reminded me that another way to force a game save on the fly is to use this simple built-in game cheat: hold down the left SHIFT key and the minus ("-") key on the numeric keypad, then type the word SAVEGAME and hit the ENTER key. You should see a message that your game position has been saved.
If for some reason you don't want to use the mission save utility nor the game cheat, you can do multiple saves manually. During SP mission play, hit ESC to pause the mission. Then return to Windows using the Alt+Tab keys and rename file SAVE.FPS in one of the following locations:
For stand-alone SP missions:
For campaign SP missions:
Later you can rename any saved copies of the file back to
SAVE.FPS should you want to revert to a particular point within a mission.
For campaign SP missions, you can also copy the SAVE.FPS file to another directory and later you can revert back to that point within the campaign by renaming it to CONTINUE.FPS and copying it back to:
before starting the game.
(With thanks to Martin Bires, of Kosice, Slovakia, for this FAQ entry).