A single diode drop is 0.6 V. So, you can see that making an op amp that works of off 0.9 V might be pretty difficult. Worse yet, at cold temperatures, that single diode drop can get up to 1.2 V. There are parts that do work at 1.2 V, the voltage of a mildly discharged alkaline battery cell.
Along these lines, the LMV951 will not only work at 0.9 V, it will even start working if that voltage is applied (Fig. 1). Texas Instruments application engineer Paul Grohe warned me, “Competitive parts do have issues. They say it will operate down to 0.9 V, but it starts up at 1.1V.”