I was looking at the options for Conjure Animal and found the Giant Owl seems way OP.

8x Velociraptor, while it may look like the Pact Tactics would get more DPR, they are more vulnerable to enemy AoE.

8x Giant Owl has Flyby attack, and hit for +3 to hit, 2d6+1 damage on hit. This means you can leave them high in the sky and swoop down for each attack.   While the to-hit is lower than the CR 2 beasts, they cap out at +7 to hit.  The max damage is 16d6+8. Even if you reduce the effective damage by 70% this is 44 damage compared with the Giant Elk's Hooves attack on a prone target of 22 damage. The elk could be killed, while the Owls are scattered overhead.

Plus fewer attack rolls to annoy the DM. Fewer creatures to micromanage. Even if you wanted to fill up the battlefield, Large creatures take up more space. If they succeed in grappling, they can fly up and drop. Giant Constrictor Snake merely keeps them put.

Rather than Fly spell, a single Giant Owl has 13 Strength. Carrying Capacity = 195 (13*15). Lifting is 2x, and large is another 2x, ie. 780 pounds.  In other words Fly for 8 members.

Giant owl understands common with an 8 Int, high enough to handle very complex tasks.

I remember seeing a discussion about summon spells, and apparently there's a debate on whether or not the summoner can choose the specific creatures that are conjured by the spell. A few people have said the wording of the spell is such that you only get to choose what class of creature appears, and the DM gets to choose what specific creatures actually manifest.

I heard that there was an errata that let it be in the hands of the player.  I forgot where.

Seems this compilation would be fairly authoritative on sage advice: https://media.wizards.com/2020/dnd/downloads/SA-Compendium.pdf . I admit that sage advice is not an errata, but a strong hint regarding the developers intentions. 

Giant Owls are OK and offer some utility, but they're not the most OP summons. They have balanced survivability and damage but where they shine is utility, being strong and big enough to carry people and smart enough to do complex stuff. The most OP thing you can do with them is to grapple people and fly directly up with them - even if the enemy breaks free from the grapple 30 feet if 3d6 fall damage, and each turn the owl could climb up 30 feet more.

Badgers are tough and get multi-attacks.

Wolves are accurate with pack tactics and can knock people prone.

Cows/Elks offer devastating alpha-strike charge.

Constrictor snakes are insanely good at crowd control.

Swarm of Ravens if your DM allows you to summon swarms of animals (spell wording is unclear on that) are great tanks.

Keep in mind you can't technically ride a flying creature without an exotic saddle.

And flying carrying capacity is not really specified and flight being heavily restricted elsewhere gives you an idea of what the designers might intend. For example, Aarakocra lose their flight entirely in medium armor. It wouldn't be unreasonable for a DM to set a fairly low flight carrying capacity or just outright say they can't lift you. Same thing for lifting and dropping - it doesn't make a lot of sense that dragging something up is as easy as dragging it along the ground. Try it with your couch.

Per [SageAdvice], the "Lifting and Carrying" rules do not distinguish between flying and walking creatures.

"Dragging" implies rough friction against a surface, so "dragging up" isn't really a thing.

The limitation on armor for flying creatures likely has more to do with the restrictiveness of bulky armor as flapping wings would lead to chaffing, and the math on high AC armor against unprotected wings wouldn't make much sense.

However, I think the RAI for the "Lift" mechanism is likely intended to mean "when braced against the ground", as it would be for "Pushing" and "Dragging". i.e. A Giant Owl should be restricted to its carrying capacity while flying in open air. Though, that's still a lot of weight to carry while flying.

I'd almost certainly implement Variant: Encumbrance rules if my players deliberately tried to exploit flying creatures in this way. That would still let a Giant Owl carry a single player with a 40ft Fly speed.

I've never screwed around with flying creatures' carrying capacity (a lot of knock-on effects once you go down that route), but I have considered the anatomy of birdlike critters to be unsuitable for serving as mounts unless they're two sizes larger than their rider. Not necessarily something I'd extend to terrestrial-shaped creatures that have wings (griffons, pegasi, dragons, manticores, etc. etc.), but just something I do for the bird-boned critters like Pteranodon and the like....

... and that doesn't interfere with them carrying things (within their weight capacity) in their claws, just stops you from piling them high as mounts. 

Well, AFAIK in Eberron Giant Owls are recognized as citizens in Shan and work as aerial taxi (and getaway drivers for criminals). It's clearly stated they can ride people on their backs, but if possible prefer to carry them in the claws or in the giant baskets they carry in the claws, because it's more comfortable. They're so common there's even a random chance the if you jump off the window or bridge (or someone helps you to) a Giant Owl would catch you and demand you pay for the ride.

Unless it's been changed, The caster is only supposed to pick the CR option for the fey creatures in the conjure animals spell and the DM chooses the beasts that appear. They can request what they want but the DM may or may not do it. You are just as likely to get any mix of: Axe Beak, Boar, Constrictor Snake, Draft Horse, Elk, Giant Badger, Giant Bat, Giant Centipede, Giant Frog, Giant Lizard, Giant Poisonous Snake, Giant Wolf Spider, Panther, Riding Horse, Wolf, etc. 

With the action economy 8x extra creatures on the battlefield is powerful no matter what. That many extra turns on one side can change the fight dramatically. 

Ewww ^^^  Then you have to have 8 different stat blocks open slowing combat way way more...  Just let them 8x their animals so they can use an online dice roller to roll 8d20.  Let them know the AC / check # so they easily know how many misses they make without a ton of effort.

The main reason for the you do not pick rule is the pixies in Conjure Woodland Beings.  They are clearly overpowered.   Owl is not overpowered. Yes, Fly By will kill in many situations but not all.

Ranged attackers will destroy them. Both archers and mages destroy them.  You can ready a strike against the Owls.

If you have certain feats, such as Sentinel (and being near an ally that gets hit) you can also attack even without reading an action.