It's too bad that most services are free in the world of Pokémon -- Professor Oak would make a fortune if he charged trainers by the Pokémon to look after them. Ash alone keeps 64 Pokémon at Professor Oak's lab, including his 30 Tauros and he's just one of many trainers from Pallet Town who do the same. With so many trainers' Pokémon living on his property, it's unclear to what extent Professor Oak is involved in their daily lives.
Trainers are only permitted to carry a maximum of six Pokémon at any time, meaning the extras must be kept somewhere. In Ash's case, his Pokémon tend to end up at Professor Oak's Laboratory. Unlike the games, the Pokémon aren't converted into data and stored on a computer. Professor Oak's property has a massive area called the Oak Corral, a diverse environment with varying habitats designed to allow all kinds of Pokémon to live comfortably.
Professor Oak might have gotten more than he bargained for when he started offering this service 25 years, seven regions and 750 Pokémon ago. Even though Ash helps lighten the load by frequently releasing Pokémon before they wind up at Oak's lab, as his journey continues, his Pokémon count continues to climb. Luckily for our hero, Professor Oak does not appear to have a restriction on the number of Pokémon that trainers may leave with him.
Pokémon are living creatures that need to be cared for, so Professor Oak can't leave the Pokémon on his property to their own devices, at least not all the time. Pokémon in the wild fend for themselves, but once caught, Pokémon are quickly domesticated and need to be fed proper Pokémon food. It's a good thing Tracey Sketchit is there to help, otherwise, Professor Oak's entire day would be spent just feeding the countless Pokémon in his charge. Hopefully, Ash's Snorlax is the only one of its kind there, considering a Snorlax needs to eat 900lbs of food every day.
Unlike Charizard's stint in the Charicific Valley, Ash's Pokémon don't usually come back from Oak's lab stronger than they were when they were left there. Sometimes they've learned new moves, but it's clear that Professor Oak doesn’t train these Pokémon. However, Pokémon Journeys establishes that some Pokémon train themselves, as Ash's Infernape took up battling Ash's other Fire-types to get stronger.
Ash's Infernape is also an indicator of how closely Professor Oak monitors trainers' Pokémon, which is to say, not very closely. When Ash and Goh return to the lab, Ash questions where his Infernape is, to which Professor Oak causally responds that he hasn't seen Infernape for a few days. So, when a trainer's Pokémon leaves Oak's property of its own volition, he apparently does not look for them. This is surely something that trainers would mention in a Yelp review.
Whenever Ash does return to visit his Pokémon at Professor Oak's Lab, they are always overjoyed to see him. Maybe they just love their trainer that much, or maybe this seldom affection from Ash's visits is the only affection they ever get. Despite the fact that most of Ash's Pokémon have spent much more time with Professor Oak than with Ash, only his Muk seems to have any affection for the professor.
None of this is to say that Professor Oak is unkind to these Pokémon, only that the sheer number of them means that feeding them is all he can reasonably accomplish on a daily basis. Considering that he is a Pokémon professor, after all, it's most likely that he spends his extra time studying them, rather than bonding with them. The unique setup of the Oak Corral means he can observe Pokémon as close to their natural habitat as possible.
Above all else, Professor Oak provides a crucial public service to help aspiring young Pokémon trainers on their journeys and asks very little in return. He is always on call if trainers want to swap out Pokémon, and he generally takes good care of the ones trainers leave with him. The real MVP of the Pokémon anime is clear, and it certainly wouldn't hurt for Ash to give Professor Oak a hand every now and then.