If you're playing an OFP version prior to OFP: Resistance version 1.75*, OFP Player Christiaan van Bladel has discovered a round-about way to help players who feel they need more ammo than the standard maximum amounts allowed by OFP at the start of any single player or campaign mission.
Once you've started a mission, press the ESC key to pause the game and then press ALT+TAB and go to the directory containing the SAVE.FPS, AUTOSAVE.FPS or CONTINUE.FPS file. In that same directory, you will also find a file named WEAPONS.CFG. As an example, here's the path where the WEAPONS.CFG file will be found, if you're playing the official SP mission "Steal The Car":
As another example, if you're playing the Red Hammer campaign, you'll find the WEAPONS.CFG file using this path:
Open file WEAPONS.CFG with any text file editor, such as Windows NotePad. Inside you will find your weapons and ammo amounts when the mission started. Your primary weapon should be the one listed as "class Item0". Just change the weapons type and/or ammo count and save.
For a full list of weapon and their matching ammo names, see this FAQ item.
After saving your changes to the WEAPONS.CFG file, go back to the paused mission and begin the mission again by clicking on REPLAY. This will cause the modified WEAPONS.CFG file to be read by OFP and be used in the mission.
If you've messed up the WEAPONS.CFG file, simply restart the mission from the SP or campaign mission list.
Have no fear! This cheat will not work for multiplayer games.
* As of OFP: Resistance, the WEAPONS.CFG file is no longer in a standard text format and can no longer be easily edited, as per the above instructions.