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.
The confirmed condition
The official Patch 1.0.10 notes say Oliver should now be recruitable when the mercy option is chosen at the start. This is stronger evidence than autocomplete, a character list, or an unsourced route summary.
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.
A clean verification run
Update to 1.0.10 or later, preserve a save before the opening mercy decision, choose mercy, record when Oliver next appears, and capture the actual recruitment prompt. The patch confirms the gate, but not every later scene or live stat.
Frequently asked questions
Is Oliver really in Sovereign Tower?
Yes. Official Patch 1.0.10 explicitly names Oliver and says he should be recruitable after the opening mercy choice.
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.