Steps:
1. Open Net Manager.
2. go to Local > Service Naming.
3. After selecting Service Naming, click on Edit menu > Create
4. Give a Net Service Name (any user defined name)
5. Select TCP/IP (Internet Protocol)
6. give the Host Name (IP Address or Host machine name)
7. Service Name : (it should be the global_name of the database)
select * from global_name
8. After completing it, go to File and save Network configuration.
9. Select the connection string created and clikck on command menu > test service.
10. Give the valid user name and password for connecting.

增加环境变量试试，假设你把oracle装入c：\oracle，就在环境变量里也加上