I have a 300 ct with a large open bottom and 2
trays on the top should I keep my oasis xl on
the top tray or in the bottom of my humi for
best results

do both and figure out which is better

Even a 300-ct space is still fairly small, especially for an XL unit. I can't see a top or bottom position making any significant difference, as long as there are slots in the trays. You should probably choose location for convenience sake - wire routing, frequent access to cigars, etc. You will be refilling or replacing the Oasis water unit pretty infrequently, so keep it out of the way.

I keep my cigar Oasis in the bottom of my large desktop and it blows through the openings in the tray and the humidity circulates through the humidor.

I keep that section of the humidor partitioned off so nothing impedes the circulation and no ciagrs get too close to the area where the humid air is coming off the oasis.