The following are known Action Replay Codes for KAO the Kangaroo on Nintendo Game Boy Advance (GBA).
These codes were also known as GameShark Codes and in most cases will work on both devices.

These codes are for the U.S. version of the game.

Master Code Must Be On
b3382a54
0052baf4
1ebc9ed3
2294fc97

Infinite Health
22d6aca5
84ec5485

Infinite Lives
33afca6c
0555db1b

Infinite Gloves
177e23b8
786b35c6

All Levels Open (Code 1)
6e48ff1a
02d9a2f6

All Levels Open (Code 2)
fb748013
6c683769
98e58503
83dd5929
5c2c310d
de8f3e46
2f736d58
66966cfe
71a4facf
a7ac8b22

Long Jump
f0b7d7e5
cc01beea

1 Hit Boss Kills
ac2678df
f43d08ed

Have Key on Snow Level
7548f31b
6921af80
ded12839
236ce159

Max Flags
b5ca23c6
990736ee

Max Coins
1e10a713
e5c0afdb

Press AR Button: Fly
bcbcd6cb
3cedba9f
f8a224a7
5c90b52b