A search for molecular oxygen has been carried out towards TMC2, L134N
and B335. We have obtained 1 x sigma upper limits to the O2/CO abunda
nce ratio of approximately 0.15, 0.19 and 0.29 in TMC2, L134N, and B33
5 respectively. These are the first limits to the O2 abundance obtaine
d towards dark clouds, and they are similar to those obtained previous
ly towards warm clouds. Chemical equilibrium models predict a value of
0.5 for the O2/CO ratio toward dark clouds if we assume the cosmic va
lue for the C/O abundance ratio (C/O approximately 0.4). This value is
larger than the limits obtained towards L134N and TMC2 by a factor gr
eater-than-or-equal-to 2. A C/O ratio different from the cosmic value
is proposed to explain the low O2 fractional abundance. In fact, chemi
cal equilibrium models give an O2 abundance consistent with our limits
and in agreement with previous estimated molecular abundances towards
dark clouds if we assume C/O approximately 0.7. Other possible explan
ations for the low O2 fractional abundances are also discussed.