"Over-the-Counter" stocks are company shares not listed on any exchange and, hence, traded only "over-the-counter." In fact, in the U.S., the big OTC stocks are generally traded on the National Association of Securities Dealers Automated Quotations System (NASDAQ). Quotes for these shares are available in the NASDAQ Stock Quotes database. For quotes on smaller OTC stocks, try the OTC Markets (including the former Pink Sheets) and/or the OTC Bulletin Board. If that doesn't work, brokers call dealers to find current prices.
Margin Rules: In the past, the Federal Reserve Board's margin rules said that a stock was was "marginable at brokers and dealers" (i.e., you could borrow the money to buy the stock from the broker or dealer who was selling you the stock) only if the stock was (a) traded on an exchange or (b) on a list compiled by the FRB. An amendment to Regulation T said the OTC stocks would now be marginable (and therefore subject to the FRB's margin rules) as of January 1, 1999 if and only if it trade on the NSADAQ. To get historical lists of OTC stocks that were marginable (and hence subject to the Federal Reserve Board's margin rules) before 1999, search back issues of the Federal Register. The lists were published quarterly by the FRB until November 1998 (the last list was effective through December 1998).