Oko gin (gin meaning silver in Japanese language) was specially minted circa 1650-1700's for a trade agreement with South Korea for the healthful ginseng herb. This herb was important at this time for curing sickness as well as other traditional applications. The Oko gin coin was pressed for this single trade deal and was minted with a higher purity (appx 80%) and silver content (up to 50%). The commonly used cho gin had a lower grade silver and only appx 20% content. The minting of the purer Oko gin was demanded by Korea for this trade deal when it was crucial for Japan to have the ginseng herb for remedies.
Vintage Japanese Coins: http://www.imes.boj.or.jp/cm/english_htmls/feature_gra1-9.htm
Ginseng site: http://kgc.or.kr/new_eng/?gclid=CN-Bp_790JECFQ0cHgodjwKcCA