Here we report ex situ XRF and grazing incidence X-ray scattering studies of two prototypical ptype OMIECs: poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), a dual component OMIEC consisting of a conjugated polymer templated on a polyelectrolyte; and pg2T-TT a single component conjugated polymer electrolyte OMIEC. For both we explore the OMIEC ion composition when the thin films are exposed to electrolytes, and at various electrochemical potentials (dedoped, naturally doped, redoped to a higher oxidation level) relating to degree of oxidation across systematically varied electrolyte concentration and pH. The quantitative determination of the potential/concentration/pH dependent OMIEC composition reveals the role of buffering and Donnan exclusion in polyelectrolyte based OMIECs containing an excess of fixed ionic charge. Additionally, massive amounts of ionic charge trapping compensated by trapped localized electronic charge is revealed in conjugated polymer electrolytes lacking fixed ionic charge. These synchrotron-based ex situ XRF measurements were complimented with benchtop XRF, XPS, and EQCM studies. Further, investigating the composition of record high conductivity acid crystallized PEDOT:PSS (crys-PEDOT:PSS) reveals the limits of Donnan exclusion and the complex composition that accompanies peak OMIEC conductivity.