Using OBD2 connection to modify vehicles cpu

I am unsure if this question has been answered or not, but is it possible to manipulate the cpu through the OBD2 Connection? Is the on-board diagnostics readonly or read/write? I've been looking at the freematics ttl kit 3. I do know you have to place the cpu into a service mode to program and would require the vehicle to be stationary. What I'm trying to achieve is changing the o2 saturation to keep the cpu from auto adjusting from an HHO device and maintain a PID for HHO production and ability for "on the fly" logging for adjustments.