which are the largest otc markets in the world?

Answer 1

By turnover or by capitalization? The foreign exchange market is probably has the largest turnover (daily $2 trillion spot + $3 trillion derivatives), but it does not have capitalization per se. Fixed-income derivatives, conversely, have very high capitalization (outstanding credit default swaps alone are valued at $14 trillion, but then there are mortgage strips, Treasury strips, caps/floors, etc.), but trading is not as lively as it is in currencies...

Answer 2

The currency market