If you are using OMM you will need to remove the device from OMM and then you can uninstall Okta, this is done from your Okta tenant in the Devices section. Also Android has a locking mechanisim with Device Administrator apps that prevents these apps from being uninstalled. Depending on your version of Andorid you may need to disable the app as a Device Administrator before you can uninstall it.
Here's how to disable a Device Administrator app:
1.On the managed mobile device, go to Settings. 2.Navigate to Security. 3.Select Device Administrator and disable Okta Mobile. 4.Under Settings, go to Applications. 5.Select Okta Mobile and Uninstall.