Hi Ricky,
in the T4 MIDI Setup in tab Transmit, normally (e. g. with template "All Parts") the Parts Right 1-3 and Left are routed to the MIDI channels 1-4 (of Port 1 = MIDI A Out).
But in this case, MIDI data from the T4's keyboard is ONLY transmitted, if the particular Part (e. g. Right 1) is switched ON.
When the keyboard Part that is used to control the Oasys is switched OFF while you are playing (via Part ON/OFF button, or by switching to another OTS or Registration), this could be the Problem ("hanging" MIDI notes on Oasys).
Therefore, I recommend to use the special Parts "Upper" and, if needed,"Lower" to control the Oasys from the T4. Switch OFF all Parts in tab Transmit an set only Upper (= right hand) to CH 1 and, if needed, Lower (= left hand) to CH 2. Now, these transmit channels are ALWAYS ON, even if T4's all keyboard Parts (Right 1-4, Left) are switched OFF.
I also recommend to release the checkmarks for CC (Control Change) and PC (Program Change) on the Upper/Lower Channels.
If you have changed the MIDI Setup, you finally have to press Exit and SAVE your work in User tab.
Hope this helps!
Best regards,
Chris