SLA vs OLA
The difference between a Service Level Agreement (SLA) and an Operational Level Agreement (OLA) is what the IT organization as a whole is promising to the customer (SLA), and what the functional IT groups promise to each other (OLA).
The SLA can state that "IT will ensure that computer equipment will be maintained". Of course that statement is a generalization that cannot be measured, so perhaps a better statement would be "There will be less than 100 lost man-hours per year due to lack of computer equipment maintenance".
The OLA will need to state everything that the functional IT groups will need to do in relation to each other to support the SLA. This will include what the server team will do for patching of the servers, what the desktop team will do to patch the desktop systems, what the DBAs will do to optimize the databases, etc, etc.
The idea is that the promise made in the SLA has to be measurable and completely supported by the OLAs that the SLA is reliant on.