And I just answered my own question... I found the following post which VERY helpfully mentions that pins 3 and 4 need to be shorted.

http://community.onion.io/topic/3165/connect-oled-expansion-and-omega-2-only

Re: oled exp pinout

I've been trying to get an ESP8266 to see the OLED expansion using these pinouts with no luck. An i2c scan cannot see it, though it can see other SSD1306 displays without issue. I am only connecting one display at a time so address conflicts aren't the issue.

The OLED expansion is working fine with the Omega2 so I don't understand what the problem could be. Any advice?