They're supposed to be paid observers.
They have as much arrest authority as a private citizen.
Also, under state law, an armed security officer's DPSST license is NOT a state statutory exemption from local bans on loaded carry under ORS 166.173. Though Beaverton has a specific exemption for DPSST guards, Portland does not for general DPSST guards (just the ones that work at banks). DPSST has recommended very strongly in the past that any armed guard working in the Portland Metro area get a CHL (also for protection against public building bans).