GPIO + Expansions

Hi guys,
I want to do the following and I would like to know if its possible before trying something that might damage the Onion.
I have an onion connected to a dock, the dock has 2 relays and an OLED.
Can I still use the GPIO's?
I assume the OLED and relays communicate using the TX and RX, so there should be no problem using the GPIOs?
The GPIO voltages are 2.8 and 2.0? Is that correct, I would assume that 2.8 is 1 and 2.0 is 0? What about no voltage? Is that read as 0 or 1 or an error? I can take these voltages directly from the Onion?
Sorry if all these questions are a little basic. I am just learning the ropes.
Thanks!

Hi @SamuelMathieson, The relay expansion and OLED expansion communicates with the Omega via I2C, which uses pin 20 and 21 (regardless of how many expansions you have stacked). So all the other GPIOs can still be used.
The voltage on the Omega is 2.7v. 0 is ground, and 1 is 2.7v. Yes, you can take these voltages directly from the Omega.

Thanks very much! Looking forward to see how things progress!