Email or Username:
Have you tried the new ShotOn for Oppo? This is available on the Google Play store and you can check it out here https://play.google.com/store/apps/details?id=com.shotonwatermarkstamp.addshotonforoppogalleryphotos&hl=en
The second one from ShotOn will add watermarks to your images. You can also download this and install it from the Google Play Store https://play.google.com/store/apps/details?id=com.shotonwatermarkstamp.addshotonforoppophotos&hl=en_US
I don't see much of anything else that you'd like or be interested in. I don't see or find any third party tools that would work for you. So sorry.
I will recommend you one stunning android app named " ShotOn Stamp On Gallery: Add Shot On Tag to Photos". This application allows you to add your Phone's Brand Name (Along with its logo) regardless of which phone do you have! It also allows you to add your name to the photos that are already saved in your device!
You can download it from here: https://play.google.com/store/apps/details?id=com.shotonwatermarkstamp.addshotonforgalleryphotos&showAllReviews=true