Well you could spend hours/Weeks/months in editing your sounds to get a uniform Volume levelPieter Meij wrote:
Also, connecting the Oasys to a mixer first makes good sense but how do you compensate for all the different volume levels. I mean if I keep the main volume slider at maximum position, I select a busy/loud combination program and I adjust the input-level gain knobs of my mixer to this setting everything works fine of course for this particular combination program which outputs the hottest possible signal. But what do I have to do if I select another program which is much softer in it's output? Would this mean that you have to adjust the input of your mixer everytime you select another program/combination?

If you simply alter the mixer input trim to the hottest level signal for every sound without clipping,(I know you perhaps would find this impractical for Live gigs,but this is where you would need to do some editing of sounds in order to avoid having to do this and make sounds within a song body if you need too switch programs more uniform,the last thing you want to be doing is modifying the Input trim half way though a song,and I've yet to use a digital mixer that allows automation of its Input trims )
you need to keep the Signal the Loudest you can get without pushing the signal in the mixer to clipping(with Oasys or any other device for that matter slider at max volume)(always check your Peak levels throughout the song to avoid any clipping especially if using eq and the like can easily push a signal above the desired level) keeping the Mixer fader set to 0db you will get the best SNR and cleaner recordings,and you should always try to match that level and keep it uniform if your processing it through any modulation or other devices in the chain that could maybe alter the volume or Quality of signal the output volume channel should always match the input volume that way you know your getting the best possible signal and it always will sound cleaner keeping Noise to a minimum
You should never have to touch the channel Fader on your mixer except to maybe lower the volume a little to balance a mix across the Mixer or in exceptional circumstances to give a little more Volume in the headroom but its best to use the Input trim rather than move mixer sliders if your require a little more volume
its amazing how many people You see who always leave the input trim set to a moderate signal volume on their mixing devices and never touch it but simply push up the channel fader to compensate for lower volume sounds and then can't understand why there mixes are of substandard quality or too Low in volume,this type of method introduces poor signal levels and a disaster for introducing unwanted noise a typical sign of amateur recordings
Using the fader to gain volume instead of using the Input trim first to maximise the signal due to poor gain staging at source is like trying to drive a car at full speed with one foot on the accelerator and the other on the brakes