Archer Pull-ups
Archer pull-ups are the direct progression toward the one-arm pull-up. Each rep, one arm does the main pull while the other extends sideways as a guide. Asymmetry increases gradually until the guiding arm contributes less and less.
Execution
Grip the bar very wide (more than double shoulder width). From the hang, pull the body toward one side bending that elbow while the other arm nearly fully extends. The bent side does 70-80% of the work. Alternate sides each rep.
Progression toward oapu
Increase the work ratio of the active arm. Over time the extended arm does less until it is only a balance aid (2 fingers → 1 finger → nothing = OAPU).
Rotation
The body will want to rotate toward the working arm. Brace the obliques hard to stay aligned.
Goal
3×5 per side with 80%+ of the work on the active arm before attempting assisted OAPU.