In short, yes.
If you look at your board, you will see everything arranged in vertical strips. You will see a microphone input and also somewhere near it, you will see a 1/4" line input. Each one of those strips is what's called a channel and each channel will be wired into a pre amp. So if you wanted to plug in an electric guitar into the line input on channel one it would go through the pre amp on channel one. A microphone plugged into channel two will be passed through the channel two pre amp, etc.
Now in Logic, you will not hear anything until you have the tracks armed to record. To record an electric guitar and a say, a vocal track simultaneously, you would create two mono audio tracks in Logic, then on the first track, you would make sure the track is recording channel one under the input tab on the channel strip and the 2nd track is recording channel two, etc. You will click the little record buttons next to each track which will light them up and arm them. Once you have the levels the way you want on the Onyx board, you will click the main record button next to play, stop, pause, etc and it will start recording everything simultaneously from the different mics and instruments.
I should note that most boards will not work with both a microphone and a line instrument plugged into the same channel and that can sometimes damage your board by overloading it. One instrument or mic per channel.
