1) Toad does not officially support Oracle 8.
2) You will need an Oracle client of the same bitness as Toad. If your Toad is 64-bit then it can only work with 64-bit Oracle clients. If you must use this old client then you will need to install 32-bit Toad. If you are already using 32-bit Toad then we need to dig deeper so send your Support Bundle text (Help|Support Bundle).
Michael