No, adding features from ArcGIS for Server to ArcGIS Online does not consume credits.
When features are published as map services in ArcGIS for Server and the service is then added as an item in ArcGIS Online, credits are not consumed. This is because the data is stored in ArcGIS for Server. In ArcGIS Online, only the REST URL of the service is referenced in the added item.
However, if a user publishes a hosted feature service in ArcGIS Online, credits are consumed from the organization as these hosted feature services are stored on ArcGIS Online.
Users can create feature services directly from the data using My Contents in ArcGIS Online by uploading the features and having them hosted as services, if they have publisher or administrator privileges in the organization.
For more information about service credits consumption, please see the link in the Related Information section.