I'm a member of Marketo's Privacy Team. We handle email deliverability and compliance, and work closely with our sysadmins to maintain the mail servers.
The designation "oob" means this is an "out of band" bounce. Normal bounces happen as part of the initial email transaction - our server tries to send, and the recipient server refuses the message. An oob bounce happens when a recipient server initially accepts the message, and then later sends us a bounce saying that the message was refused. Regular email bounces arrive a standard format, but an oob bounce comes in the form of an email message sent by the receiving system. These don't follow a standard format, and it can confuse our mail servers and make it impossible for us to find the original bounce information.
It's a bit of a confusing concept, so I hope this was clear. The end result for you is that we know we received a bounce, but we are not able to learn any more about it because our automated systems cannot interpret the information that was sent to us.
As far as the Spamcop bounces you're getting, we're doing our best to reduce the impact of those issues by moving customers who cause these problems to quarantine servers and to make some changes to our internal architecture. If you send over 300k messages per month, you should look into a dedicated IP for your mailings. On the other hand, if you send significantly below this number and you are confident that you have excellent sending practices, you can contact the support team and ask to apply for our Trusted IPs. Trusted IPs are free of charge for low volume customers who meet strict requirements around data collection and sending behavior.