Attempting to combine or diplex OTA and CATV signals, if done improperly, will cause harmful interference to licensed radio services such as police, fire, ambulance, rescue and aircraft communications and navigation frequencies.
CATV systems are designed as 'closed systems'. This means that no signal should be radiated over the air. Connecting an antenna to a CATV system will result in radiation of signals on frequencies that are used by legitimate licensees. You could be placing lives at risk and would certainly be at risk of civil or even criminal penalty.
Instead of trying to combine the two incompatible signal types, consider buying a tuner/converter. Some are available with high definition (HD) output.
Keeping the CATV cable completely separate from the OTA cabling, leave the CATV signal connected to the TV antenna input as it is now. Then, connect the external tuner/converter to an HDMI port or HD composite input on the TV. (The TV must have one or both, or it likely does not have HD capability.) With the OTA antenna connected to the external tuner/converter you'll have access to both CATV and OTA without risk of one causing trouble with the other.
See the following thread for a diagram and some possible tuner/converter options:
http://forum.tvfool.com/showthread.php?t=2882