OPSEC refers to the level of ease at which the opposite party (e.g. the company whose system is under scrutiny) can discern activity of an operation.
In the context of red teaming, during a red team engagement, operators must adjust their actions according to OPSEC, leave minimum traces, and prevent detection. This concept can also apply to the blue team, since the red team may have compromised monitoring implements (SIEM, ticketing system, response procedure documentation, etc) and render the blue team completely oblivious to malicious activities. In any case, it would be wise to assume that the opposing side is more advanced than one’s own.