The CO postcode area, also known as the Colchester postcode area, is a group of sixteen postcode districts in the east of England, within nine post towns. These cover northeast Essex (including Colchester, Clacton-on-Sea, Frinton-on-Sea, Halstead, Harwich, Manningtree and Walton-on-the-Naze) and a small part of south Suffolk (including Bures and Sudbury). The main sorting office was in Colchester but as at 2021 there was no Mail Centre in Colchester.
The approximate coverage of the postcode districts:
|- ! CO1 | COLCHESTER | Colchester | Colchester |- ! CO2 | COLCHESTER | Old Heath, Berechurch, Layer de la Haye | Colchester |- ! CO3 | COLCHESTER | Lexden, Fordham Heath, Stanway | Colchester |- ! CO4 | COLCHESTER | Greenstead, Highwoods, St Johns, Myland, Boxted, Braiswick | Colchester |- ! CO5 | COLCHESTER | Tiptree, Kelvedon, West Mersea, Peldon, Rowhedge | Colchester, Braintree, Maldon |- ! CO6 | COLCHESTER | Coggeshall, Earls Colne, Marks Tey, Great Tey, Chappel, White Colne, Wakes Colne, Fordham, Copford, West Bergholt, Great Horkesley, Wormingford, Nayland, Stoke-by-Nayland, Polstead | Colchester, Braintree, Babergh |- ! CO7 | COLCHESTER | Brightlingsea, Wivenhoe, Great Bentley, Alresford | Colchester, Tendring, Babergh |- ! CO8 | BURES | Bures, Alphamstone | Colchester, Braintree, Babergh |- ! CO9 | HALSTEAD | Halstead | Braintree |- ! CO10 | SUDBURY | Sudbury, Clare, Lavenham, Long Melford | Babergh, West Suffolk, Braintree |- ! CO11 | MANNINGTREE | Manningtree, Lawford, Mistley, Bradfield, Wrabness, Wix, Horsley Cross, Little Bromley, Brantham, Cattawade | Tendring, Babergh |- ! CO12 | HARWICH | Harwich, Dovercourt, Parkeston, Ramsey, Little Oakley, Great Oakley, Stones Green | Tendring |- ! CO13 | FRINTON-ON-SEA | Frinton-on-Sea | Tendring |- ! CO14 | WALTON ON THE NAZE | Walton-on-the-Naze | Tendring |- ! CO15 | CLACTON-ON-SEA | Clacton-on-Sea, Jaywick | Tendring |- ! CO16 | CLACTON-ON-SEA | Clacton-on-Sea, St Osyth, Little Clacton | Tendring