Hi Saurabh,
Most often at a minimum, all you need to utilize ArcGIS GeoEvent Server is an installation of ArcGIS Server (licensed as GeoEvent), and then an installation of GeoEvent Server on the same machine. This type of set up should be sufficient to allow you to read from ArcGIS Online feature services, and subsequently write to ArcGIS Online feature services. Its not necessary that you install and configure a Portal + other supporting ArcGIS Enterprise elements. Its important to note however that stream services (from GeoEvent Server) cannot be published/hosted in ArcGIS Online. They have to be published to the underlying ArcGIS Server since they run as a part of the GeoEvent Server service (to really generalize things here). You can certainly add the stream services to your ArcGIS Online organization as items (just as you would with a non-hosted map/feature service), but the stream service still needs to be published & running on the GeoEvent Server machine.
If you are primarily working with ArcGIS Online already, you don't have a need for an on-premise Portal (or ArcGIS Enterprise for that matter), nor do you want to deal with the infrastructure aspects of managing GeoEvent Server, we do offer a new product called ArcGIS Velocity which could likely fit your needs. Velocity is a new capability of ArcGIS Online that provide real-time and big data processing & analytic capabilities. With it, you can ingest real-time data, perform on-the-fly analytics, and even output it to feature/stream services hosted in ArcGIS Online (and more). Since ArcGIS Velocity is a SaaS, there's nothing for you to manage in terms of infrastructure. Everything is hosted in ArcGIS Online. Something worth considering given your questions!
Hope this helps!
