<Page> EXHIBIT 12.1 OLD SECOND BANCORP, INC. RATIO OF EARNINGS TO FIXED CHARGES <Table> <Caption> AS AND FOR THE THREE MONTHS ENDED MARCH 31, AS OF AND FOR THE YEAR ENDED DECEMBER 31, ------------------------------------------------------------------------------------ 2003 2002 2002 2001 2000 1999 1998 ------------------------------------------------------------------------------------ INCOME BEFORE INCOME TAXES PLUS: $8,023 $7,567 $30,897 $26,486 $19,425 $18,188 $16,085 INTEREST EXPENSE 6,435 6,588 28,009 35,290 35,816 29,154 32,229 OCCUPANCY EXPENSE 849 676 2,855 2,771 2,581 2,431 2,358 ------------------------------------------------------------------------------------ EARNINGS 15,307 14,831 61,761 64,547 57,822 49,773 50,672 ------------------------------------------------------------------------------------ INTEREST EXPENSE 6,435 6,588 28,009 35,290 35,816 29,154 32,229 OCCUPANCY EXPENSE 849 676 2,855 2,771 2,581 2,431 2,358 ------------------------------------------------------------------------------------ FIXED CHARGES 7,284 7,264 30,864 38,061 38,397 31,585 34,587 ------------------------------------------------------------------------------------ RATIO OF EARNINGS TO FIXED CHARGES, INCLUDING DEPOSIT INTEREST 2.10x 2.04x 2.00x 1.70x 1.51x 1.58x 1.47x ==================================================================================== INCOME BEFORE INCOME TAXES PLUS: $8,023 $7,567 $30,897 $26,486 $19,425 $18,188 $16,085 INTEREST, NON DEPOSIT 188 324 952 2,165 1,606 1,938 2,866 OCCUPANCY EXPENSE 849 676 2,855 2,771 2,581 2,431 2,358 ------------------------------------------------------------------------------------ EARNINGS 9,060 8,567 34,704 31,422 23,612 22,557 21,309 ------------------------------------------------------------------------------------ INTEREST, NON DEPOSIT 188 324 952 2,165 1,606 1,938 2,866 OCCUPANCY EXPENSE 849 676 2,855 2,771 2,581 2,431 2,358 ------------------------------------------------------------------------------------ FIXED CHARGES 1,037 1,000 3,807 4,936 4,187 4,369 5,224 ------------------------------------------------------------------------------------ RATIO OF EARNINGS TO FIXED CHARGES, EXCLUDING DEPOSIT INTEREST 873.67% 856.70% 911.58% 636.59% 563.94% 516.30% 407.91% ==================================================================================== </Table>