Oxidized can be configured via the web GUI in Global Settings → External → Graylog Integration
To override the GUI and force the config (not editable via the GUI anymore) configure in config.php
#----------------#
# Graylog Config #
#----------------#
$config['graylog']['server'] = 'http://grlg1.int.mydomain.net';
$config['graylog']['port'] = 9000;
$config['graylog']['username'] = 'graylog-user';
$config['graylog']['password'] = '*****************';
$config['graylog']['version'] = '2.5';
$config['graylog']['timezone'] = 'America/Denver';
$config['graylog']['match-any-address'] = 'true';