Sanji [OP06-119_r1]

My opponent played "OP06-119 Sanji" when they had 0 Life cards, and used its [On Play] effect to play "OP08-069 Charlotte Linlin". Then, after the effect of [Charlotte Linlin] added 1 card from their deck to their Life cards, can I activate this Leader's [Once Per Turn] effect to make them add 1 card from their Life cards to their hand?

Yes, you can. You can choose not to activate the [Once Per Turn] effect of [Koala] when Sanji is played, and then choose to activate it in response to [Charlotte Linlin] being played after your opponent has activated and resolved the [On Play] effect of [Charlotte Linlin].