No. You can play Oil Empire as a guest without creating an account. An account is optional and is mainly used for cloud saving and accessing your progress across supported devices.

Open the account section in the game, choose Sign Up, enter your email address and password, and follow the instructions shown.

Your current supported game progress can be associated with the new account and uploaded as its cloud save.

Yes, where supported. Sign in to the same account on the other device to retrieve the progress associated with that account.

The progress stored on the account may replace the progress currently stored on your device. Oil Empire displays a warning before this happens.

Signing in to an existing account can replace your current local progress with the account's cloud save. The warning is there to prevent accidental loss of local progress.