I installed a whale Supersub 650 on my Oasis. For simplicity, and so I am in control of when the pump turns on, I went with a switched model rather than automatic. The pump is mounted to the hull bottom just forward of the aft seat hatch. I used heavy duty clear plastic tubing rather than the cheap leaky black corrugated stuff as it lasts forever and you can see what is going on or if water is trapped in the system. I mounted the drain as high as I could facing out to the right of the aft seat. I did not install a check valve since they do not work that well (slow seepage if inflow), at least the small cheap versions used in kayaks, they can prevent proper pump priming especially with centfrigal pumps, and if mounted at all below the outlet water can collect and remain in the system. Instead I installed an adjustable snaplock brass and rubber drain plug on the outside of the outlet, with a cord to a pad eye so it doesn't get lost. To pump I just remove the plug and flip the power switch which is also mounted beside the seat, and I can easily see the water leaving the outlet and when it stops. As it turns out this Oasis is much dryer in the hull than either of my Outfitters so I have only used the pump thus far for testing and the occassional flushing of the hull interior with fresh water.
What switch are you using ?