  Answering a question posed by Conway and Norton in their seminal 1979 paper
on moonshine, we prove the existence of a graded infinite-dimensional module
for the sporadic simple group of O'Nan, for which the McKay--Thompson series
are weight $3/2$ modular forms. The coefficients of these series may be
expressed in terms of class numbers, traces of singular moduli, and central
critical values of quadratic twists of weight 2 modular $L$-functions. As a
consequence, for primes $p$ dividing the order of the O'Nan group we obtain
congruences between O'Nan group character values and class numbers, $p$-parts
of Selmer groups, and Tate--Shafarevich groups of certain elliptic curves. This
work represents the first example of moonshine involving arithmetic invariants
of this type.
