Can I capture different native resolutions with the same CCD-Sensor?
No, there is physically only one 'native resolution' for any hardware sensor. It is the number of physically distinct light sensitive areas on the sensor chip and is set in silicone at the time of manufacture.

When OBS or any other software gives you a different sized image it is either re-sampling using the values available from the native resolution or alternatively changing the camera resolution which then puts the burden of re-sampling on the cameras internal processing chips.

(Unless you are working with RAW output then the information lost between using a native resolution and sub-sampled resolution is going to be swamped by what is thrown away by modern compression techniques such as JPEG anyway.)