how to write a condition for all elements of a list

You could also try AllTrue:

If[AllTrue[a, # == yes &], Print["that's OK"]]

Not sure what you're trying to do, but for your specific example, the following works:

If[Union@a == {yes}, Print["that's OK"]]

An even more expressive approach (in my opinion at least) is to use ContainsOnly:

If[ContainsOnly[{yes}]@a, Print["that's OK"]];
(* that's OK *)