Two problems with (o arg default) - you need to remember not to use 'o at the start of a destructuring list (and not get confused when you see (def handle-request ((i o ip)) ...) ), and as akkartik says it's paren-inefficient, a single keyword to delimit required/optional args would mean fewer tokens.
The first problem is easy to fix though - use a symbol that's less likely to be an arg name to identify optional args. How about '= ?
(def myfun (a b (= c (something)) ...)
it has the advantage of similarity with ruby:
def myfun a, b, c=something
disadvantage: looks silly when you don't supply a default value: