Otc is over the counter trading (ie getting an entity to trade with you at a set price rather than doing a market buy). It just means don't buy off exchanges, find other sources as they should be more reliable (especially if you have multiple).
[ https://medium.com/circle-research/how-does-crypto-otc-actually-work-e2215c4bb13 ]
It seems that OP what looking for which seems trying to compete with this kind of service. But one thing that I saw is some serious problems that you will experience OP --this OTC trading cannot be automated by using an API just like an exchange platform, the same as mentioned above, it could have more terrible fees than the regular exchange which seems worst. The last one is, it is good for large-scale traders, not small-scale traders.
Here is a link if you will need OTC brokers where to connect API.
[ https://www.sfox.com/blog/13-bitcoin-otc-brokers-you-need-to-know/ ]