Woo, WoO, WOO, W.O.O. and variants may refer to:

• To woo, a term for courtship
• A term for pseudoscience, attributed to James Randi.
• WoO (Beethoven), ("Works without opus number"), a cataloging system for Beethoven's work
• Woo (film), a 1998 romantic comedy
• Woo Woo, an alcoholic beverage
• WOO, the callsign for AT&T High Seas Service
• Wooooo!, onomatopoeia of excitement, a cheer
• Woo, a character in the King of the Monsters series of video games
• Bio Planet WoO, a Japanese television show
• Woooo! (How I Met Your Mother), a 2008 television episode
• Ronnie Woo Woo, a fan at Chicago Cubs baseball games
• Kitchen Party (group), a British girl group formerly known as The WooWoos
• Woo (Ultra monster), a fictional monster in the Ultraman television series
• Woo, a character in The Big Lebowski
• Woo or Wu, romanization of several East Asian names:
  • Hu (Chinese surname) (胡,瓠,護,戶,扈,虎,呼,忽,斛)
  • Wu (Chinese surname)  (吳,伍,武,仵,烏,鄔,巫)
  • Ng (Chinese surname) (吳,伍)
  • Woo (Korean name), Korean surname and given name (우)
• College of Wooster
• World of Outlaws, a car racing organization
• Any of the various adaptations of The Wizard of Oz
• "Woo", a song by Rihanna from Anti

("Works without opus number") (WoO), also Kinsky–Halm Catalogue, is a German musical catalogue prepared in 1955 by Georg Kinsky and Hans Halm, listing all of the compositions of Ludwig van Beethoven that were not originally published with an opus number, or survived only as fragments.

Woo! is the first release by Omaha, Nebraska-based band Tilly and the Wall. It was self-released in 2003 with 6 songs, recorded in Conor Oberst's basement studio.