The Great Horned Owl is one of the most easily recognized owls in the area. Fairly common in the Yosemite area, these owls hunt rabbits, rodents and even other owls, and have been commonly seen in the hollows of Black Oak trees near the base of Yosemite Falls.
Most years there is a Great Horned Owl nest in at least one meadow in the Valley. It's great to get a glimpse and watch these guys, but keep in mind that observers near their tree will keep the mother from hunting and the owlet or owlets may die, as happened to one in Cook's Meadow in 2011. The nest was quite near one of the most popular paths in the Valley and, though the reasons for the nest failure can only be guessed, the traffic likely did not help. So get a nice pair of binoculars and observe from a long ways away, surreptitiously and briefly.
Great Horned Owls are highly territorial and mating pairs will attack and kill other owls and daytime raptors. In return, they will be mobbed by other birds. They can be seen perching during the day, often hardly moving. They'll commonly use the big stick nests of Red-tailed Hawks, but in Yosemite Valley they are most commonly seen in the holes in oak trees as in the pictures below.
Further reading: David Lukas, Sierra Nevada Birds, p. 93–94.