The basis of otc bitcoin is the average price between exchanges. Different exchanges have different prices in different countries. The best way to understand price summation is to look at the kimchi special. The price of bitcoin in Korea was much lower than in the United States because bitcoin was illegal to buy in America, Which resulted in people taking their dollars to Korea to buy bitcoin. After bitcoin was legal to buy in the US, The price averaged out, And the kimchi special disappeared. This is how Sam Bankman-Fried founded FTX. OTC price is just an average between exchanges. You can still buy on peer to peer boards like Bisq. You could offer someone cash to get bitcoin. The examples of how to acquire bitcoin are endless.