The invention provides a process to form a composition comprising at least one ethylene-based polymer and at least one oil, said process comprising the following: polymerizing a mixture comprising ethylene, and optionally one or more comonomers, in a reactor system, comprising at least one reactor and at least one mixer, located downstream from the reactor, to form the ethylene-based polymer; and wherein the oil is added to the ethylene-based polymer upstream of the mixer. The invention also provides a reactor system for a process to form a composition comprising at least one ethylene-based polymer and at least one oil, said reactor system comprising at least one reactor, at least one mixer, and an oil feed into the ethylene-based polymer, and wherein the oil feed is located after the reactor and before the mixer.