Based on one of OFP's officially supplied SP missions, "Clean Sweep", the new OFP Demo version 1.42, released on March 12 2002, also includes a new mission name "Skirmish". Instead of playing a spec ops soldier carrying an MP5, you're a standard NATO solider with an M16.
You can download it from here (349KB). Simply unzip the demo's PBO file into OFP's MISSIONS subdirectory. I've already renamed the PBO file so that it will correctly play in OFP's full retail version.