Your O3 is a function of t. You have O3^m where m is an unknown.
You are not going to find a solver that is able to deal with that situation -- not unless it runs through a few special cases and leaves the rest unresolved.
syms O3(t) m k kla O3x
for m = [-5 -4 -3 -exp(sym(1)) -2 -sqrt(sym(2)) -1 -sym(1)/2 -sym(1)/3 0 sym(1)/3 sym(1)/2 1 sqrt(sym(2)) 2 exp(sym(1)) 3 4 5]
disp('----')
m
ode = diff(O3)+k*(O3^m) == (kla*(O3x-O3))
dsolve(ode, 'MaxDegree',4)
end
