What you want is not straightforward. OTA and cable frequencies overlap (you should check that), so you cannot mix them into a single tuner.

Solutions?
Buy a TV with two tuner inputs, or
If you want a single or few channels from OTA, there are amplifiers that can inject a single channel into broadband cable. Televes has some model with this capability, with an adjustable bandpass filter. You have to check that the channel you want to inject does not overlap any signal from cable.