@defragster: Just wanted to make sure we were talking about the same core. Yes, the Oak and ESP8266 Arduino cores do share a lot of toolchain stuff - which is easy to see if you install one and then the other, as the Arduino IDE doesn't bother downloading the full toolchain, just the extra unique files needed for the core you are installing.
I run different versions of the Arduino IDE side by side - 0022, 1.0, and then every version from 1.6.5 to 1.6.8, and haven't had any issues with 1.6.6 to 1.6.8 with Oak, other than the same "Board oak1 (platform oak, package digistump) is unknown" error the OP reported, which was when I was running 1.6.7, and appeared to be connected with the change from 0.9.4 to 0.9.5 of the Oak core. Manual cleanup and reinstall of the Oak core fixed that. My upgrade from 1.0 to 1.0.1 when seamlessly this morning, so that was a good sign!