OneIdentity is an attribute that can be assigned to a symbol f to indicate that f[x], f[f[x]], etc. are all equivalent to x for the purpose of pattern matching.
Functions like Plus and Times have the attribute OneIdentity.
The fact that Times has attribute OneIdentity allows a pattern like n_. x_ to match x.
See The Mathematica Book: Section 2.3.7 and Section 2.5.3.
See also: Flat, Nest.