OTC is one of the great mysteries. I seem to remember asking repeatedly on here what everyone's experiences were with them and only got a couple of answers back.
Should I need to cash out I'd vastly prefer to use somewhere with tailored service rather than some asshole exchange ready to freeze me and turn my future into a ticket number. I want to hear more about them before considering it though.
I've made a research on some known OTC platforms, and here's what I've found regarding their trading volume
Cumberland Mining
Cumberland claims to have done over $20 billion volume in 2017 alone.
Circle
itBit
itBit’s OTC trading desk trades between 15,000 - 25,000 XBT per month.
CoinCola
By analyzing the BTC-CNY transaction data of LocalBitcoins, Paxful and CoinCola in the past two weeks, the total transaction volume of the three platforms was 680 million yuan [around 100 mln USD], of which the Hongkong-based CoinCola increased significantly, accounting for up to 70%. [OCT 30, 2017]
And finally, an estimation cited by Forbes [Oct 23, 2018]:
A report earlier this year produced by the Tabb Group estimated daily bitcoin OTC volumes at $12B globally.
I don't know if all of the above is true, but that's what I've managed to find, and it would be interesting to see the comments regarding how close to reality those numbers are.