Our three didactum environmental sensor boxes failed to be discovered correctly. Observium just showed them as "generic linux devices" and therefore did not query the private mibs correctly.
The problem was with os discovery because the system Object ID reported by the devices was .126.96.36.199.4.1.46501, not .188.8.131.52.4.1.8072.3.2.10 as stated in the os-detection include file.
Solution was to change the file /opt/observium/includes/definitions/os/didactum.inc.php to:
$config['os'][$os]['discovery'] = array(
'sysObjectID' => '.184.108.40.206.4.1.8072.3.2.10',
'sysDescr' => '/Didactum EMS/',
After a new discovery, the boxes where shown correctly with the private mibs enabled.