OTC #Bitcoin means over the counter Bitcoin.

There are companies that hold large amounts of Bitcoin and they sell it in bulk to you.

That means; if you want to buy 10 million dollars worth of Bitcoin in one go, they have the Bitcoin on the shelve. And they can provide you with that huge amount for a set price.

This means you don't have to go on an exchange. You don't want to buy that amount on an exchange, because you will influence the price to your own disadvantage. You will be pumping your own buy price. Hence, you want to buy OTC bitcoin in bulk.

This is what Blackrock does and where all the big players buy their Bitcoin. It's to not pump the price.

This chart shows; the OTC sellers have ran out of Bitcoin... if you want bulk BTC now, you will have to pump the price on an exchange.