For executing Oracle SQL queries and PL/SQL Code, you can refer to the websites below:
Oracle Live Sql - you can save all your PL/SQL scripts
Sql Fiddle - you can create schemas and queries and share them with other users.
Oracle Apex -you can test out Apex, which can let you create objects and run scripts.
Sql Zoo - It is a PL/SQL coding ground to create objects and run scripts
Tutorials Point- Here also you can create objects and run scripts
You can execute .sql scripts by using the following steps.
1.Click on new project.
2.Click on '+' symbol. -- A new .sql file will be created (Newfile.sql)
3.Execute the .sql as @filename.sql (Ex : @Newfile.sql )