Considering all the odd and fantastic creatures that turn up in Oz, it's not too surprising that there would be some giants, is it? There are a few in the Baum books, and a LOT in Thompson's additions to the series. Here are some of the more notable appearances of giants in the Oz books:
First of all, the Yoops, perhaps the most famous giants in Oz. In The Patchwork Girl of Oz, Ojo's party comes across the cage of Mr. Yoop, advertised as the largest untamed giant in captivity. According to his identification card, he's twenty-one feet tall, weighs 1640 pounds, and has the typical ogrish habit of eating human flesh, which he prefers with orange marmalade. Even though he's caged, he still poses a threat to passersby, as the corridor in front of his prison is narrow, and he can reach pretty far. In Tin Woodman, the protagonists come across Yoop Castle in the Gillikin Country, and assume it's unoccupied. As it turns out, though, it's still the home of Mrs. Yoop, giantess and expert in transformations.
The giant slaves of the super-strong people of Herku.
Crinklink, a giant ferryman with the ability to change size and talking jacket buttons. He imprisons Dorothy, but later reveals himself to be the Wizard trying to teach her a lesson.
The giant subjects of King Terribus of Spor, on the Enchanted Island of Yew. Considering that the Red Rogue grew to be bigger than any giant, yet he could still walk around a castle built for humans, I have to suspect that these giants aren't quite as big as the Yoops. Still, they're feared fighters.
The inhabitants of the Giants' Peak, who have an unusual arrangement of eyes, with one in the forehead and one in the back of the head. They feature in the story "The Littlest Giant," which is really only linked to Oz by its subtitle "An Oz Story" and a brief reference to the Gillikins. It's suspected that Baum might have added these bits in as marketing tools. I'm currently reading Chris Dulabone's The Giant King of Oz, which better ties in the Giants' Peak with the Oz series, and also features Mr. Yoop as commander of the giants' army.
Bangladore, the Candy Giant, who's addressed in this post.
Ruggedo, the former Nome King, who's normally pretty small but used Glegg's Instantaneous Expanding Extract to turn himself into a giant three-quarters of a mile tall during Kabumpo. In this form, he ran away to Ev with Ozma's palace stuck on his head. In the same book, Tik-Tok mentions a three-legged giant living in Ev.
Crunch, a stone man three times the size of an ordinary person, animated by the wizard Wam and later de-animated by Glinda and the Wizard of Oz.
The Big Wigs of Immense City in Ev, who are only giants while they wear their wigs. See this post for more details.
Nandywog, who was kicked out of the giants' colony on Big Top Mountain for never growing to be more than twenty feet tall. He settled in the triangular Gillikin valley of Tripedalia, where all the people and animals have three legs each. The Tripedalians are willing to accede to Nandywog's every request, but they're afraid of him, and the giant longs for companionship. When Randy and Kabumpo visit in Purple Prince, they befriend Nandywog, although the elephant is secretly jealous of pretty much everyone bigger than he is. Is Big Top Mountain the same as the Giants' Peak? Probably not, as there's no indication that Nandywog has the peculiar eye arrangement of those giants. Melody Grandy has indicated, however, that she thinks of her Huge Mountain as the same place as Big Top, only with a different name to stave off copyright issues. Her own giant protagonist, Orlando, becomes king of this mountain.
Loxo the Lucky, a giant who seems to be about the same size as his home of Big Enough Mountain, and possesses a magic magnet that will attract anything he desires. When King Sizzeroo accidentally runs Umbrella Island into his head in Speedy, Loxo demands restitution in the form of a boot-lacer, as he finds this task difficult to perform himself. The wizard Waddy tricks him into eating a sponge cake that shrinks him down to normal human size, but he's apparently satisfied with this. According to a Fred Otto short story in Oziana, he settles down to become a farmer in Grapelandia. The Umbrellians read about Loxo in the Encyclopedia of Giants, which is a book I wouldn't mind seeing.
Snorpus the Mighty, whom the Wizard of Wutz employed to guard the entrance to the Silver Mountain. Snorpus has only one eye, which rotates between four eyeholes. When Handy Mandy tricks him into letting her into Wutz's lair, the wizard reduces him in size and plants him in a flowerpot, apparently a common punishment in Wutz's domain.
Terp the Terrible, who grows to giant size by eating from the magic muffin tree in the Hidden Valley, and then sets a monstrous guardian at the tree to make sure no one else eats the muffins. The Tin Woodman cuts down the tree, and his companions trick Terp into the smokestack of his jam factory, where he presumably shrinks back down to human size.
Badmannah the Terrible, a rather uncouth black-bearded cave-dweller on Upandup Mountain in the Gillikin Country. Like Loxo, he also has a magical tool, in his case a drag net. Unlike the fairly sedentary Loxo, however, Badmannah uses his net to steal castles, which he then despoils of their treasures, keeping their princesses as slaves. Jinnicky deals with him by commanding his own drag net to send him to the middle of the Nonestic Ocean.
The gypsy giants who are passing through the Winkie Country at the time of Enchanted Island. One of them, Jerry, tries to keep the camel Humpty as a plaything. Giants apparently pass through the area often enough for Nick Chopper to employ a watchman named Ben Tover to look out for them.