HPAO may refer to:

• AOC3, Amine oxidase, copper containing 3 (vascular adhesion protein 1)
• N1-acetylpolyamine oxidase, an enzyme

• April 26, 2007
• April 27, 2003
• April 27, 2004
• April 27, 2007
• April 28, 2003
• April 28, 2004
• April 28, 2007
• April 29, 1987
• April 29, 2003
• April 29, 2004
• April 29, 2007
• April 30, 2003
• April 30, 2004
• April 30, 2007
• April 3, 2003
• April 3, 2004
• April 4, 2004
• April 5, 2003
• April 5, 2004
• April 5th Action
• April 6, 2003
• April 6, 2004
• April 6, 2039
• April 6-7, 2006 Tornado Outbreak
• April 6-7 2006 Tornado Outbreak