I dont know exactly when that trademark O&S was used and its a very unusual way to hallmark gold one would think,but i cant help feeling its familiar,ive seen those markings before.
Either on relatives or friends wedding bands possibly although i cant imagine how id see those which makes me think an older relative maybe a grandparent.
Of course i could be wrong , it just seems familiar.
It is most likely an artisan crafted ring. A one-of-a-kind ring that many times were marked "solid gold" as the exact gold content was not known, and was not required by law to mark it's exact purity.
The makers' mark is very similar to that of the Danish jewellers Ottesen & Sundstrom that were active in the first quarter of 20th century. "Solid Gold" hallmark is very common (starting from the end of 19th century) but vague. It can be anything from gold to gold-filled.