OLA stands for 'Operation Level Agreement', UC stands for 'Underpinning Contract'. In practice, these are just labels for different types of SLA-type agreements. The functionality is the same regardless of your choice here so if you don't need to define one vs the other just use SLA.
Typically OLAs are internal organization-organization agreements and SLAs are with an external company.
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See Also
what is Underpinning contract ?