Is it possible to create external table in Snowflake referring to on premise Oracle database?
No, Snowflake does not presently support query federation to other DBMS software.
External tables in Snowflake exist only to expose a collection of data files (commonly found in data-lake architectures) as a qualified table without requiring a load first.
Querying your Oracle tables will currently require an explicit export of its data onto a cloud storage location to allow Snowflake to access it.