Off Blockchain transactions
Just as the name implies, off-blockchain transactions are transactions during which the cryptocurrency value moves outside of the blockchain. Such transactions are especially popular between large participants because their cost is usually zero or very low.
Open source
A term used to describe the software with "open", publicly accessible code. Anyone could inspect it and modify it at will, as well as use it in one's own projects.
Orphaned block
A block that wasn't accepted into the blockchain due to various reasons; the main reason is usually a time lag occurring during the procedure of acceptance. The other term used for such blocks is "detached" because they are isolated from the blockchain.
OTC exchange
"OTC" stands for "Over-the-counter." The term refers to the process of security trading for the companies that are not listed on a formal exchange such as the London Stock Exchange (LSE). Securities that are traded over-the-counter are traded via a broker-dealer network as opposed to on a centralized exchange.
Output
Outputs are sets of instructions for coin spending. The value of outputs equals the total amount of coins claimed. The sum of outputs must always be equal to the sum of inputs, no matter how many outputs there are (transaction fee is excluded).