Direct answer
As of Patch 1.0.10, Oliver should be recruitable if you choose the mercy option at the start. Older New Game+ saves could remain locked into earlier decisions because of bugs fixed in that patch.
Opening recruitment condition
Choose mercy at the opening and keep a save before that decision. Continue until Oliver's next appearance, then use the recruitment prompt when it becomes available.
Why older routes failed
The same patch fixed New Game+ cases that incorrectly locked the player into previous decisions and repeated quest outcomes. A save created before the patch may therefore behave differently from a fresh, updated test.
Current-build checklist
Update to 1.0.10 or later, preserve a save before the opening mercy decision, choose mercy, and record when Oliver next appears. If an old New Game+ save repeats the previous outcome, retry from the pre-choice save.
Frequently asked questions
Is Oliver recruitable in Sovereign Tower?
Yes. From Patch 1.0.10 onward, choose mercy at the opening and continue to his later recruitment opportunity.
Why can I not recruit Oliver in New Game+?
Older builds had New Game+ decision-lock bugs. Update the game, then test from a save where the opening mercy option is selected.