At least one optional key present in clojure's Schema

You can specify whatever predicate you want in your schemas:

(def my-schema
  (s/both
     (s/pred (complement empty) 'not-empty)
     {(s/optional-key :foo) Bool
      (s/optional-key :baz) Bool
      (s/optional-key :bar) Bool}))

If you are asking how to validate the map using just the built-in predicates you could write:

(def my-schema
  (s/both
     {(s/optional-key :foo) Bool
      (s/optional-key :baz) Bool
      (s/optional-key :bar) Bool}
     (s/either
      {:foo Bool s/Any s/Any}
      {:baz Bool s/Any s/Any}
      {:bar Bool s/Any s/Any})))

which is pretty ugly and a lot more verbose than the pred example