I know you were just using it as an example, but I wanted to point out the command should be:
su oracleuser -c "/usr/local/bin/oraclient"
Posting the script would be useful. Otherwise, you'll get vague, generic responses like this:
The PATH might not be what you're expecting within the init script. Do a which su
and provide the full path in your script. For instance, on my machine, it would look like:
/bin/su oracleuser -c "/usr/local/bin/oraclient"
If /usr/local is located on a separate partition, verify that it has been mounted before the script tries to run oracleclient.
Other than that, I'd need to see the script before I could offer any other suggestions (that is, *if* I could offer any more).