Do you mean UNcorrectables or correctables? Lots of correctables on OFDM is normal because of the way OFDM uses FEC. Even up to 100% correctables can be considered normal. But UNcorrectables on any channel and any modulation is never normal. If you have UNcorrectables it means there's a problem with the data getting to your modem. You should never disconnect or connect the coax cable with the modem powered on. This will cause a large number of uncorrectables.
As far as I know Cox only supports 1 downstream OFDM channel and 0 upstream OFDM channels at this time in all areas. 1 OFDM channel is more than enough to support fill gigabit speeds.