Copy oeupdatemaker and oeupdatemaker% to /PSP/GAME if you are in firmware 1.50, or to
/PSP/GAME150 if you are in 2.71 SE or 3.02 OE.
Get the 1.50 and 3.03 sony updates, and put them in the oeupdatemaker folder with the names
150.PBP and 303.PBP.
Run oeupdatemaker. It will generate the dxar file DATA.DXAR, whhich has the following hashes:
MD5: 03186D46D2F1B0ACD923098BEBC87281
SHA1: 3C80833A970EEA16A94D74DAFF6220889A9CEE4A
Copy 303oeflasher and 303oeflasher% to /PSP/GAME if you are in 1.50, or to
/PSP/GAME150 if you are in 2.71 SE or 3.02 OE.
Move the DATA.DXAR file to the 303oeflasher.
Run the program. When it finishes, it will ask you to reinit the psp. When you do that, you will be in 3.03 OE-A.