Operation codes originate from the Bitcoin Script language which pushes data or performs functions within a pubkey script or signature script.
A pubkey script is included in transaction outputs and sets the conditions that must be fulfilled for those satoshis to be spent. Data for fulfilling the conditions can be provided in a signature script. Pubkey Scripts are called a
scriptPubKey in code.
A signature script is generated by a spender. Signature scripts are almost always used as variables to satisfy a pubkey script. Signature Scripts are called
scriptSig in code.