Missing the 17 meter (18 mhz range) and 12 meter (24.9 mhz range) WARC crystals from your Omni C? If your display reads 9 mhz when you turn to those bands, your crystal is either bad or not installed. That was an option and was not purchased with some radios. The suggested crystals in the links below should work with your radio but I cannot be held responsible if they do not. If one is out of stock, a similar one can be ordered and tried.
Since Ten Tec is no longer offering any parts for these radios you can order a 4.000 mhz clock crystal from Mouser, Digikey, or other supplier and that will activate the 18 mhz band for under a $1 less shipping. Since the leads on this replacement cryrstal are wire and not a pin like the original crystal, you can salvage a HC49 crystal with the larger pins, desolder the case, remove the quartz crystal and then carefully tack solder the wires to the pins. Grounding the case is optional as there is no grounding on the OEM crystal. Then plug it in.
The 10.5 mhz crystal that was used for the 12 meter band is almost impossible to find. I would suggest looking at the various 10.7 mhz range crystals that are offered and substitute this instead. Then follow the same procedure to install as above. Moving the hetereodyne oscillator crystal frequency to 10.7 mhz will shift the display reading to around 24.7 mhz. Since the radio uses a digital readout it will “compensate” for the difference in frequency depending on the frequency of your crystal. The original tuning range was 24.5-25.0 mhz and with a 10.7 mhz crystal it is now 24.7 to 25.2.
https://www.mouser.com/ProductDetail/IQD/LFXTAL003171Bulk?qs=e4%2FAndAAwgKYd%2F1b1N%2Ftgg%3D%3D
Your radio may require alignment of the RX RF AMP capacitor for 18 mhz as mine did not come aligned from the factory and was fully meshed. The bandpass filter for 18 mhz was pre-aligned in my radio. Refer to the service manual for servicing information. The 12 meter band shares the same capacitor adjustment as 10 meters and no adjustment should be necessary for it.