Thanks for reaching out, I am so glad you found us!
If the ring is 80 years old, it was likely manufactured around 1946. Vintage hallmarks often use elaborate fonts or symbols, and discontinued purity standards, like 12-karat or 15-karat, were still in circulation during this era.
18 is likely part of the karat mark and indicates that the ring is 18-karat yellow gold.
OI might be a sponsor's or retailer's mark, or another possibility, could also be a regional import stamp.
