Can someone explain this error?

[eluser]JoostV[/eluser]
This looks like a bug.

I looked into my 1.7.1 install and found that the constant OCI_COMMIT_ON_SUCCESS is used in system/database/drivers/oci8/oci8_driver.php, but it was never defined anywhere. Hence the error.

Code:

// Set "auto commit" by default
var $_commit = OCI_COMMIT_ON_SUCCESS;

Set it beforehand and you should be fine. Or, it may be fixed in 1.7.2 or SVN trunk.