This monogram is from a 14th century Manuscript Alphabet book which resides in The British Museum in London, England. 

Monogram:  O

Accents/Accessories: Rolled Edges; Jump ring bail; Central monogram

Size: 40.64 x 22.86 millimeters (1.60 x 0.90 inches)

Weight: 6.80 grams (0.24 ounces)

Sterling Silver Chain as shown: 16 inches - $25.00 or 18 inches - $35.00