I am using VB6 application on Windows 7 and I created DSN using Microsoft ODBC Driver for Oracle but I got the above error when I try to connect to Database.
Please help me out on this that why this error is coming
Based on what I'm seeing in the docs, Oracle 11 client can't connect to Oracle 8.
ODAC 10.2.0.2.20 could, provided the server is 8i release 3 or higher. That said, I don't think it has support for Windows 7 and since it was released well before Windows 7 was I'm not sure it'll even work.
TBH at this point Oracle 8 server is pretty archaic and you probably need to look at migrating.
Provided its a 32-bit application, possibly. Microsoft ODBC for Oracle is discontinued and has no 64-bit version, so a 64-bit application won't be able to use it at all.
I do see it listed as a 32-bit ODBC driver on my Windows 7. The problem is that I think it relies on having an Oracle client installed on the machine, and you've got the same problem of Oracle client 11 not working with 8 server. You could try it and see if it works I suppose, or try installing an older client and see if that makes it work.
If it's just for testing purposes though you could throw up an Oracle XE server and test against that.