With regards to the "O" standing for Overlander...that's based on a faulty document that has floated around in the Airstream archives for years and states the "0" or "O" stands for Overlander which was a 26' Airstream (available in both the Land Yacht & International versions). It is wrong and keeps being perpetuated as it is impossible to purge it from the internet. It mistakenly indicates a "0" (zero) as an "O" (letter). It stated the "O" (Oh) stands for Overlander when in actuality it is intended as a "0" (zero) which = Land Yacht package versus "I" (eye) or "1" (one) = International package.
The International package had a few extra bells & whistles. "0" (zero) is just a placeholder when the "Land Yacht" level of finish is present. "I" was used for "International" finishes.
The term "Overlander" has always stood for a length of trailer, not a level of finish.
I concur with reinergirl's assessment of what your trailer is based on the posted VIN #0228s163:
0 or O = Land Yacht package
22 = 22' long (Safari)
8 = built in 1968
S = built in Santa Fe, CA
163 = sequential number
Good luck with your new-to-you '68 22' Safari! Check out the archives
for some additional examples to confirm your model....you ought to see alot of similarities!