Oracle doesn't support a SELECT without a FROM statement in pure SQL.

To achieve the same behavior as SELECT 1; in Oracle, simply use Oracle's native dummy table, called DUAL, i.e., do SELECT 1 FROM DUAL;.

Related docs:

https://docs.oracle.com/en/database/oracle/oracle-database/19/sqlrf/Selecting-from-the-DUAL-Table.html

Note: On Oracle 23+, it is now optional to select expressions using the FROM DUAL clause

https://docs.oracle.com/en/database/oracle/oracle-database/23/sqlrf/Selecting-from-the-DUAL-Table.html