证明:过点E作EG‖BC交AC于点G,
∵EG‖BC,∴AE:AB=AG:AC
又∵AB=AC,∴AE=AG,∴BE=CG
又∵BE=CF,∴CG=CF,
∵EG‖OC,∴FO:OE=FC:CG=1:1
∴OE=OF