Off-chain

A transaction that occurs outside of a blockchain. Such transactions have zero/low cost.

Omni layer

Omni is a platform for creating and trading custom digital assets and currencies. It is built on top of Bitcoin.

Order book

An order book is used for almost every exchange for various assets such as stocks, cryptocurrencies and more. Represents a list of orders with different offers from buyers and sellers.

One Cancels the Other Order (OCO)

When two orders have been simultaneously created but only one can be executed.

Orphan block

A block with an unknown parent block created during older versions of Bitcoin Core.