NASA's Cosmic Background Explorer (COBE) was launched into a polar orbit from the Vandenberg Air Force Base, California on November 18, 1989. The COBE conthin three scientific instruments. Two of these are infrared instruments housed within a 660 liter toroidal superfluid helium cryogen tank. The tank is designed to maintain the base of the instruments below 1.6 K for the duration of the planned one year mission. Boil-off helium is vented from the cryogen tank through a porous plug liquid vapor phase separator, and then overboard from the spacecraft. We discuss here the initial thermal set-up and operation of the dewar in general, and the helium vent system in particular. During the initial cooldown of the dewar from 1.72 K to 1.41 K, short term (1 mm ≤ t ≤ 3 mm) temperature and pressure oscillations were observed in the porous plug and in the vent line. These oscillations have continued throughout the mission life. A detailed flow model was developed to describe this phenomenon and is described below. We further detail the slow establishment of a steady state, 'mission mode' operation of the dewar. The various factors leading to a two week time to mission mode equilibrium for the dewar and cryogenic instruments are discussed. Finally we summarize the performance of the dewar and instruments through the first six months, and we project the expectations for the remainder of the mission through the final depletion of the liquid helium.