Volume: The number of trades in a security over a period of time. On a chart, volume is usually represented as a histogram (vertical bars) below the price chart. The NYSE and Nasdaq measure volume differently. For every buyer, there is a seller: 100 shares bought = 100 shares sold. The NYSE would count this as one trade and as 100 shares of volume. However, the Nasdaq would count each side of the trade and as 200 shares volume.

http://stockcharts.com/school/doku.php?id=chart_school:glossary_v

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As can be seen in the table, the (equal-weighted) mean turnover on Nasdaq (174%) is about twice as large as on NYSE and AMEX(87%). This discrepancy reflects the fact that Nasdaq is a dealer market where volume can be double counted. Some researchers therefore divide Nasdaq turnover by two to make it compariable to NYSE and AMEX.

http://websearch.cs.com/wm/boomframe.jsp?query=how+is+trading+volume+calculated&page=1&offset=0&result_url=redir%3Fsrc%3Dwebsearch%26requestId%3De6100ace7362b710%26clickedItemRank%3D8%26userQuery%3Dhow%2Bis%2Btrading%2Bvolume%2Bcalculated%26clickedItemURN%3Dhttp%253A%252F%252Ffaculty-gsb.stanford.edu%252Fnagel%252Fpdfs%252FVolume.pdf%26invocationType%3D-%26fromPage%3DWMTResultsT%26amp%3BampTest%3D1&remove_url=http%3A%2F%2Ffaculty-gsb.stanford.edu%2Fnagel%2Fpdfs%2FVolume.pdf

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I believe OTC follows the Nasdaq in calculating volume but am not 100% sure.

Regards,

PxP