Just keep banging away at the other girls quests and eventually Olga will contact you. If I recall advancing her questline involves building the bungalows for her and Rick tries to sabotage it. For me getting Grace, Mi Sun and Rachel were the last girls.i'm stuck with olga quest,
it is explained to get closer to olga and olga's point are already max 75 but there is no event to trigger next relationship status.