When you said there is a bar below the horizontal O, you said there was another bar below the O. If the second bar is actually above the O, then it is most likely a 10K mark with part of the letter K obscured from wear. If my assumption is correct, then this would be the mark used by Ostby Barton of Providence, Rhode Island.
Engelhart Cornelius Ostby was born in Oslo, Norway and emigrated to the US in 1869. In 1879, he founed his jewelry business in partnership with Nathan Barton.
Mr Ostby was one of the 1500 passengers who perished on the Titanic ship in 1912. In 1914, his daugher Helen and her brother Harold became joint owners of the company which eventually ceased jewelry production in the 1950's. Ostby Barton items are highly desired by fine antique jewelry collectors due to their exquisite workmanship.
