Formed in silica aerogels, positronium annihilation in oxygen and nitrogen mixture is studied by a simple and convenient method. The results show that ortho-positronium （o-Ps） lifetime is significantly shortened when some oxygen is introduced into ni- trogen gas and that the o-Ps collisional quenching rate for an oxy- gen molecule is （28.0±0.4） ±ts%amagat（1 amagat 2.69± 1025 m-3）. It is found that the o-Ps annihilation rate in oxygen of latin at room temperature is （25.5±0.5） us-1 and that the effective num- ber of electrons per oxygen molecule available for 27 annihilation of positron in the o-Ps is （34.6±0.4）.
