Starting June 2018 Google has implemented a pay-as-you-go pricing model for the Google Maps Platform APIs. Previously, most users with moderate usage could use Google Maps, which are embedded into opCharts and Open-AudIT Professional/Enterprise, for free. Google's new policy is outlined HERE: https://developers.google.com/maps/documentation/javascript/usage-and-billing
In order to enable Google Maps you will need to a Google API Key: https://cloud.google.com/maps-platform/#get-started
Once you have your Google API key you will need to add it to opCommon.nmis to enable Maps in opCharts. You can find opCommon.nmis in /usr/local/omk/conf/opCommon.nmis, edit the file with vim (or another suitable text editor) and search for the opcharts_map_api_key property.
Open Open-AudIT Professional/Enterprise and select Admin -> Configuration -> All from the menu. Search for maps_api_key and place your API key in the value position.