Perhaps the Studbook keeper wanted the younger Sumatrans to have priority and I presume that he/she would have approved Oscar's move.
Incidentally, I don't think that Chester would move such an old orang now: they do not always respond well to transfers.
I imagine Chester had the choice, keep his daughters and breed from them, or send them away and keep Oscar. Oscar got moved. I think you are right they would not do this nowadays.
I remember seeing the adult male 'Benjamin' the day after he arrived at Dudley from Ireland. He was extremely nervous and would not move from one 'safe' spot. He had bad diarrhoea too, indicating his mental state. I've also heard of adolescent males 'pining' in new surroundings to the extent they have died or had to be returned to their previous Zoo. 'Batu' at Twyross spent several months hiding away under sacks before he adjusted to his new home.