So after some additional research I ended up on the Vintage Airstream site and saw that the "O" stands for Landyacht. The following script was pasted from their site.
Ohio | J O(I) 24 5 NNN
California | S O(I) 24 5 NNN
(5NN suffix for Double-bed layout)
(O for Landyacht options, I for Intíl options)
First year the serial numbers adopt a common format between the factories.
It is not hard to see why the 1965
model year is considered to be one of the best. The Tradewind example is the double-bed layout with lots of Honduran Mahogany, solid steel appliances, Zolatone walls (last year) and a beautifully designed fiberglass bathroom