The GHO's in my neighborhood constantly surprise me with sounds I didn't know they could make. If I hear a weird sound outside at night, and it's not identifiable as an animal I recognize, then it's probably a Great Horned Owl. (Except when it's a bat or other owl species, which are not frequent.) I don't know what any of these sounds mean to the owls. Here is what I have heard so far:
"H-hooo, hoo-hoo"- (Also "Hoo-h-hoo, hoo-hoo" & other variations) Most frequent stereotypical owl sounds, varies by individual, or perhaps their mood, season, weather, or other factors. Seems like an individual sticks with their own sound, almost always with a skip in the pattern, but others will sometimes alter their own and sing overlapping, or in unison with another owl. Pattern usually repeats about 20 seconds apart. Sometimes one will give just one or two hoo's.
Raspy sqeak or squawk- Single loud, screechy sound, similar to Barn Owl. Most frequently heard last spring when young were fledging, but still being fed. They still make this sound, however, at end of summer, but not as often. They used to do it all night, repeated about every 20 seconds.
Whistle- Loud whistle, 1-2 seconds long, rising or falling, seemingly random sound, not musical. Infrequently done.
Barks- Sounds like a dog you've never heard in your neighborhood before. Single bark, without the resonance of real dog barks. Repeats for a few minutes, not all night as dogs will do.
Yawls- Sounds like a cat yawling, but single, with less variation in sound, repeated a few times.
"Eyuuu-whUP"- Say this loudly in a cartoon character voice, rising in pitch at the end. Also can be a raspier version of this sound. Repeats several times, about 20 seconds apart.
"Phee-yuu"- High pitched sound, like that of a Northern Flicker, but softer and briefer.
"Brrrrrrrrrrrrrrr"-Gentle, rolling purr, heard twice at close range (shortly before I passed a tree and came nearly face to face with a Great Horned Owl).
