Fake owls might keep some pests away, for a limited time. Animals that are the prey of large owls instinctively recognize and avoid owl shapes. The problem is that these animals aren't stupid. They quickly learn that the owl is not, in fact, any threat at all and ignore it. Some birds respond to owls not by fleeing but by mobbing the predator as a group, which is rather counterproductive if your aim was to scare the birds off your property. The animals that are potentially scared of owl statues are those with good eyesight that the owl might eat, including birds and small mammals.
Rabbits are most likely to be scared of owl statues that resemble large owls in their region. Most owls are too small to take adult rabbits, but the great horned owl is a threat to both juveniles and fully grown bunnies. Get down to rabbit eye level and choose several highly visible locations for the statue. Move the statue at least once a day until you have installed a rabbit-proof fence or other barrier.
As with rabbits, pigeons may be intimidated by an owl statue for a few days, especially if you move it around. Position the fake owl close to but at a much higher level than whatever you wish to protect. For example, if you want to keep pigeons away from your car, place the statue on a roof overlooking the car.
Crows learn quickly, and an owl statue is likely to work for an even shorter period on them than other animals. If you merely need to keep crows off your garden for a day, a fake owl might work. Crows might mob the owl, but this keeps them away from your fruit bushes or seeds while you install a more permanent barrier.
Seagulls are too big to be intimidated by owls. They do view owls as a threat and may well mob them, but an owl is unlikely to keep seagulls away for long, if at all. The only real solution to seagull problems is to ensure people don't feed them and that garbage is securely sealed in cans. An owl might make seagulls think twice about hanging around, but its main purpose will be decoration.
An owl might work well on small birds such as sparrows if you need to scare them away for a while. Fake owls may also scare squirrels. There are few other mammalian pests that owl statues would frighten. Most mammal pests are either too large to be prey for owls, have poor eyesight, or are nocturnal and wouldn't see the statue. Mice might be the main food of many owls, for example, but a plastic statue will have no effect on them.