Numbers from "The Little Schemer" in Clojure

numbers.clj

(defn add1 [x] (+ x 1))

(defn sub1 [x] (- x 1))

(defn o+ [x y]
  (cond (= y 0) x
        :else (o+ (add1 x) (sub1 y))))

(defn o- [x y]
  (cond (= y 0) x
        :else (o- (sub1 x) (sub1 y))))

(defn o* [x y]
  (cond (= 0 y) 0
        (= 1 y) x
        :else (o+ x (o* x (sub1 y)))))

(defn o-div [x y]
  (cond (= 0 y) nil
        (= 1 y) x
        (< x y) 0
        :else (add1 (o-div (o- x y) y))))