Campbell
Relaxed Intensity
As I understand it agency is a measure of your ability to impact your environment. Autonomy is your ability to choose what you will do. If we are playing a game and there are no meaningful constraints on what any player (including the GM) may choose to do based on the actions we choose to take than we have perfect autonomy and no real agency. If we cannot affect each other reliably and be vulnerable to consequences for our choices than no real agency exists.
Those constraints do not have to come from resolution mechanics, but they do have to be there in some way. We can be constrained by our prep, by the shared fiction, etc. We get to this in OSR play by instructing the GM to be a neutral arbiter and referee.
My use of agency here is coming from the dictionary meaning of the word. Not some RPG theory lexicon.
www.merriam-webster.com
Here's autonomy for comparison
www.merriam-webster.com
Those constraints do not have to come from resolution mechanics, but they do have to be there in some way. We can be constrained by our prep, by the shared fiction, etc. We get to this in OSR play by instructing the GM to be a neutral arbiter and referee.
My use of agency here is coming from the dictionary meaning of the word. Not some RPG theory lexicon.

Definition of AGENCY
the office or function of an agent; the relationship between a principal and that person's agent; the capacity, condition, or state of acting or of exerting power : operation… See the full definition
Here's autonomy for comparison

Definition of AUTONOMY
the quality or state of being self-governing; especially : the right of self-government; self-directing freedom and especially moral independence; a self-governing state… See the full definition