6 files declare their use of OEmbedInterface in Drupal 8

media.install in core/modules/media/media.install

Install, uninstall and update hooks for Media module.

media.module in core/modules/media/media.module

Provides media items.

OEmbedForm.php in core/modules/media_library/src/Form/OEmbedForm.php

OEmbedFormatter.php in core/modules/media/src/Plugin/Field/FieldFormatter/OEmbedFormatter.php

OEmbedResourceConstraintValidator.php in core/modules/media/src/Plugin/Validation/Constraint/OEmbedResourceConstraintValidator.php

OEmbedWidget.php in core/modules/media/src/Plugin/Field/FieldWidget/OEmbedWidget.php