What is ob in a stock symbol?

An .OB extension indicates a NASDAQ bulletin board stock, which can include smaller companies that don't meet the listing requirements for the regular stock exchange (a listed company needs to be at least certain size, and have at least a certain number of shares outstanding at a specified price).

Since a .OB stock is not listed the stock will be followed by fewer people and only have limited interest but it still has to file regular forms with the SEC, which gives a measure of safety, though certainly not as safe as listed stocks.