removing a oscar cilchid from tank ????

Although an advocate of U/G filters you've hit upon the one situation which I would say is a definite 'no'.

These filters rely upon the gravel for a build up of bacteria in and around the actual gravel itself - but with a complete gravel bed.
Oscars are renowned for digging holes in the gravel, it's part of their job description.
If your Oscar creates a pit in the gravel to the extent that the plate can be seen then the water being drawn into the plate will all take the easiest route and by-pass the gravel. It doesn't take the valuable bacteria very long to be starved of oxygen-rich water passing over it and it quickly dies off.
I hope I've made this understandable (well, I understand it, but that's not really saying much!).
So, to answer your question: do not use U/G filters for large burrowing Cichlids...Oscars are quite 'messy' eaters, I've found so I would suggest the largest internal, or better still, external filter you can afford, you can always restrict the filter but you cannot increase its flow.

As to catching your fish, another option worth considering is after your tank light is switched off leave it for an hour or so and then switch it on again (make sure the room the tank is in is in complete darkness too).
You should then be able to catch the Oscar, which should be pretty dopey for a few moments when the light goes back on - before it regains its composure!
If you have a black plastic bucket (I use the cheapo ones from B&Q) you could 3/4 fill that with tank water and hold it in that while you are making the tank changeover. Makes sure you have a fairly heavy item to put over the top as a lid though - Oscars are pretty strong lads - so you might want to put a heavy item on your makeshift lid as it will make a bit of a fuss for a while!

Hope that's of some help,

John