ITT Space Systems Division, along with our partners at NASA LaRC, Atmospheric and Environmental Research Inc., and the University of New Hampshire, are developing an O2 Laser Absorption Spectroscopy (LAS) instrument to satisfy the requirements of the Active Sensing of Carbon Dioxide (CO2) over Nights, Days and Seasons (ASCENDS) mission as set forth by the National Research Council's Decadal Survey. The purpose of the O2 lidar is for measuring atmospheric pressure to enable conversion of CO2 column measurements to CO2 mixing ratio. The team has chosen the 1.27um band of O2 to make these measurements in order to take advantage of reduced atmospheric backscatter, and low sensitivity to relative humidity and temperature. In 2007 ITT succeeded in demonstrating a set of low power horizontal ground-based O2 measurements using the same receiver used for the CO2 system. The existing instrument has been developed and operationally validated for CO2 measurements through multiple airborne field campaigns since 2004, using a combination of investments by ITT and NASA. Since the initial measurements of O2 ITT has been internally funding the development of an advanced fiber Raman amplifier, in conjunction with the University of Arizona, to produce the powers required to demonstrate this measurement from and Aircraft and to scale it to space. A description of the instrument, the technique being used for these O2 measurements, current developments regarding the fiber Raman amplifier and results of initial testing will be presented.