An opportunity attack provoked by moving out of a threatened square takes place immediately before the movement (the trigger). Opportunistic Withdrawal helps you to avoid provoking any OAs from the adjacent enemy granting CA caused by any movement on your part. Note that if the creature had reach and you tried to move from 10' away to 15' away (after moving from 5' to 10' with no OAs) you would provoke an OA since you are not adjacent when the trigger occurs.
Opportunity Attack (4e Compendium)
Interrupts Target’s Action: An opportunity action takes place before the target finishes its action.
In this case, the triggering action should be considered the part of your movement going from 5' away to another square 5' away (circling the enemy) or 10' away (disengaging from the enemy). Continuing to move away from the enemy (10' away to 15' away) would be considered a separate triggering action.