This guide will let you know where to get Oberon Parts in Warframe. Go through the guide and follow the instructions to get the parts.
You can farm Oberon parts by completing points of interest in certain Proxima’s in Railjack missions.
The Earth Proxima points of interest rewards can drop Oberon’s systems, Saturn can drop his Chassis, and the Veil Proxima can drop his head/Neuroptics.
Once you get into a Railjack mission in any of the 3 Proxima’s, you’ll know there is a point of interest in your mission because you will have a particular varying objective that appears under the objectives that you would normally get every mission.
His parts don’t have the lowest drop rates, so you should be able to get them in some runs or a few runs if you’re lucky.