OLAP stands for On-Line Analytical Processing and the more common term for this is "Cubes". From this structure, one can "slice and dice" the data to present information in a multi-dimensional format. For example, if you want to see the efficiencies for various equipment, then you can use the dimensions of time (hourly, daily, weekly, monthly, etc.), the process or plant area, type of unit operation, etc.
Historians are the source of this information that will typically provide such data including averages and aggregated data. Historians though provide time stamped data in flat files, and the data needs to be extracted, transformed and loaded into these cubes. The data can be then viewed accordingly.
This data can be also combined with data from other sources to provide a true holistic and integrated view related to business, operations, maintenance, process control etc.