The work proposes two novel spreading codes, called extensive double-orthogonal code (EDOC) and flexible paired-orthogonal code (FPOC), for multi-user downlink multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. The goal of code designs is to obtain an improved bit error rate (BER) performance without loss of bandwidth efficiency. The code designs achieve space diversity by employing multiple antennas at both transmit ends and receive ends. Codes are reasonably spread into time, frequency and space domain. The proposed codes are attractively characterized with length flexibility, which breaks the "power of two" constraint. The simulation results indicate that the proposed codes improve overall BER performance, by exploiting space diversity gains. The flexibility in code length also results in high spectral efficiency.