There is no way to retrieve your master password if you forgot it.
It is advisable to store the master password in a safe place. Also make sure to back up your data regularly (using menu > backup or MyBackup Pro). If you have exported your data previously (using menu > export) and copied oisafe.csv to a safe location, then you can retrieve the passwords from the oisafe.csv file by using menu > import.
You can reset the password by uninstalling and reinstalling OI Safe. Note that this will delete all previously stored items.
You can also clear the data associated with OI Safe and start over. Home -> Settings -> Applications -> Manage Applications -> OI Safe -> Clear Data.