It can drop from any Eximus unit, so there is no specific system that you would need to farm - instead you should focus on high level missions. Interception missions are specified in the quote below as, if done right, you can prolong them for a very long time, assuming a good group, to ensure high amounts of Eximus spawns.
From the WARFRAME-wikia page on Oberon:
All Eximus units have a 1% chance to drop a blueprint, and if they do, it's a 38.72% chance each for Oberon's Helmet and Chassis, and 22.56% for his Systems. Hence the total probabilities are 0.3872% and 0.2256% respectively, resulting in 605 expected required Eximus kills to obtain the full set. Therefore, in order to explicitly search Oberon components, it is recommended to do missions with high Eximus density, like Sorties or high level Interception missions.
You can not trade non-Prime Blueprints except for Nezha, so trading with higher level players is not an option (thanks to MacedonZero for the heads-up).