See SDO-UTIL documentation. Methods such as SDO_UTIL.TO_GEOJSON or SDO_UTIL.TO_JSON_VARCHAR might fit. For example:
SELECT
B.ID
, B.GEOM
, CAST(SDO_UTIL.TO_GEOJSON(B.GEOM) AS VARCHAR2(500)) GEOJSON_TEXT
FROM
MYTABLE B
would return the GeoJSON in a CLOB then casted to VARCHAR2. The results would look as next:

There's a specialized forum for Oracle Spatial and Graph here https://community.oracle.com/community/groundbreakers/database/oracle-database-options/spatial which has already a lot of question and answers to basic and complex issues.
Disclaimer: I work for Oracle Spatial team; however, anything and
everything expressed or stated here are my own personal opinions and
do not necessarily represent my current and/or former employer’s view.