Hey, yeah you will need a compass and baro for the omnibus and also a baro in order to use return to home functionality. However that said if you are going to be using the position hold and return to home functionality it is best to use an external compass (usually mounted on the GPS module). So it's best not to get a FC with compass built in.
So assuming you have the omnibus version with the baro, all you need to do is connect the GPS to one of the UART ports, and then the external co.pass to the i2c port and you are good to go (after configration + calibration)
Also one thing to note, since I last checked a couple months ago, the software only allows one i2c device to be uned, so if your FC has a built in compass (usually not good enough for position hold due to interference) you will need to physically remove it before you can use an external compass