He hasn't just done well in Portugal though. He had an excellent short loan at Villarreal, in Spain, and he was bossing it there as well, constantly retaining the ball, and his passing accuracy was through the roof like 90+% as a 19 year old.
Again, the only reason why Atletico keep loaning him out is because have Koke and the way they play now, they don't have a use for Oliver's skill set at the moment.
But I have a feeling that's going to change here soon judging from the players Simeone has signed (continues to win my admiration with every passing season). A 4-3-3 might suit them more since Martinez came from that system and if so, that opens up a spot for Oliver.
The truth of the matter is Iniesta and Rakitic, at least for two seasons, aren't going anywhere and they will be Lucho's go to pairing. Xavi only rotated those two and in truth, that's all his replacement needs to do at the time being as well. Oliver would have at least 2 seasons to grow into his role here and he could take Xavi's role and make it his own here. I feel exactly the same way about Rafinha taking over for Iniesta, he just needs to stay patient, more so than his brother, and bid his time. If he keeps progressing the way he has (he has a big chance before the January replacement can play), he can be exactly what we need as well.
Investing in your youth when you have a true engrained style that won't change from manager to manager will always one way or another pay off.