Welcome to Android Central! So you forgot the master password you created for OI Safe? Probably the only thing you can do is send an email to the developer and ask them what options you have: support@openintents.org. And hope that the developer is still active -- the last time that app was updated was July 2017. Good luck!
In the future, either use a password safe or write your passwords down and stick them in a sock drawer. (If you're doing it right - a completely different password for each app and each website - you'll either need a small notebook or a password safe.)