Which version of ArcGIS Desktop are you working with?
Beginning with ArcGIS 10.7 and ArcGIS Pro 2.3, you cannot create user-schema geodatabases in Oracle.
If you are on an older version check this: Create a geodatabase in Oracle—Help | ArcGIS Desktop (the link shared is for 10.6)
Once the master sde geodatabase exists, you can use any of the aforementioned tools or scripts to create a user-schema geodatabase.
- Create Enterprise Geodatabase: When the database administrator runs this tool, be sure the schema owner's name is specified for the geodatabase administrator.
- Enable Enterprise Geodatabase: If you do not have access to the sys user's password, the database administrator can grant you the privileges required to create a geodatabase. Then you can connect to the geodatabase with your login and run the Enable Enterprise Geodatabase tool or script to create a geodatabase in your schema.
For connecting to an User Schema Geodatabase: Connect to a specific geodatabase version—Geodatabases in Oracle | ArcGIS Desktop
By default, the Database Connection dialog box in the Catalog window connects to the DEFAULT version of the master sde geodatabase. If you have user-schema geodatabases or use geodatabase versioning, you might want to save a connection to a different version.
To save a connection to a user-schema geodatabase or a version other than DEFAULT, follow these steps:
- Right-click your existing database connection in the Catalog window in ArcMap or ArcCatalog and click Geodatabase Connection Properties.
- Choose what type of geodatabase version you want to connect to:
- Choose Transactional version to connect to a user-schema geodatabase or a transactional version other than DEFAULT.
- Choose Historical version to connect to an historical marker or specific date and time.
Be aware that user schema Geodatabase creation will be deprecated.
See the full document in the link below
is there already any experience existing with migration paths to move away from user schema geodatabase f. e. to Oracle pluggable database? What to take into account? How to treat archiving, versioning, etc.?