The following article also applies to most other OM-D and Pen models but with less Fn buttons to choose as alternate AEL/AFL button. The OM-D E-M1 II and PEN-F have updated menus but in the end configuration is similar.
Many photographers prefer to decouple exposure meturing from AF but struggle how to permanently set up this behaviour on their E-M1. The reason is that the E-M1 like all other Olympus cameras simply offers different AEL/AFL modes for S-AF, C-AF and MF, but doesn't call the relevant ones AF-on. As a consequence, if you search the manual for AF-on or AF on demand, you don't find anything although the feature is available.
Olympus often publishes updated versions of the manuals when new features have been added to the cameras via a FW update.
You can download the most recent version of the manuals for the E-M1 I and II via the following download links: