In[25]:= Clear["Global'*"];
ka = .2; kd = .02; km = 150; ns = 10; sens = 3; ro2 = 50; a = .2;
solution = NDSolve[{
cella'[t] ==
ka nut[t] cella[t] - kd (1 - (1 + (nut[t]/ns)^(sens))) cella[t],
celld'[t] == kd (1 - (1 + (nut[t]/ns)^(sens))) cella[t],
o2'[t] == ro2 (1200 - o2[t]) - km cella[t],
nut'[t] == -ka nut[t] cella[t],
cella[0] == 10, celld[0] == 1, o2[0] == 1200, nut[0] == 200},
{cella[t], celld[t], o2[t], nut[t]}, {t, 0, 31}]
Out[27]= {{cella[t] -> InterpolatingFunction[{{0., 31.}}, <>][t],
celld[t] -> InterpolatingFunction[{{0., 31.}}, <>][t],
o2[t] -> InterpolatingFunction[{{0., 31.}}, <>][t],
nut[t] -> InterpolatingFunction[{{0., 31.}}, <>][t]}}
I think there were two problems. Your initial condition for nut was give as n[0]. Also your intermediate variables, odeca, etc. were not explicit functions of time and problem variables.