Can I run DDL in pretend mode in Oracle
No need to check for existence. Just wrap your statement with an exception handler that ignores away any ORA-00955 it encounters (or modify for whatever exception number you want to ignore):

DECLARE
  ObjectExists EXCEPTION;
  PRAGMA EXCEPTION_INIT(ObjectExists,-955);
  sql_stmt VARCHAR2(100) := 'create table my_table (column_1 int)';
BEGIN
  EXECUTE IMMEDIATE sql_stmt;
-- Ignore ORA-955 errors (object already exists)
EXCEPTION WHEN ObjectExists THEN NULL;
END;