Ostwald　ripening　is　a　key　mechanism　for　sintering　of　highly　dispersed　metal　nanoparticles　in　supported　catalysts.　However,　our　microscopic　understanding　of　such　processes　is　still　primitive.　In　this　work,　the　atomistic　mechanism　of　the　Ostwald　ripening　of　Cu　on　CeO2(111)　is　examined　via　density　functional　theory　calculations.　In　particular,　the　detachment　of　a　single　Cu　atom　from　ceria　supported　Cu-n　(n　=　2-10,　12,　14,　16,　18,　and　20)　clusters　and　trapping　on　the　CeO2(111)　surface　is　investigated　in　the　absence　and　presence　of　CO　adsorption.　It　is　shown　that　the　adsorption　of　CO　on　Cu　reduces　its　detachment　energy,　which　helps　in　the　formation　of　single　atom　species　on　CeO2(111).　In　addition,　the　Cu-1-CO　species　is　found　to　diffuse　on　the　CeO2(111)　surface　with　a　much　lower　barrier　than　a　Cu　atom.　These　observations　suggest　an　efficient　mechanism　for　the　Ostwald　ripening　of　Cu　clusters　supported　on　ceria　in　the　presence　of　CO.　It　is　further　predicted　that　the　Cu-1-CO　species　can　eventually　migrate　to　a　step　site　on　ceria,　generating　a　stable　single-atom　motif　with　a　relatively　larger　binding　energy.　Finally,　the　single　Cu　atom　catalyst　is　shown　to　possess　high　activity　for　the　oxygen　reduction　reaction.