I was able to get this style Heltec OLED working with I2C after struggling with it for a few hours.
I found that the OLED was physically configured as Jools suspected, in 4-wire SPI mode. The BS0, BS1, & BS2 Pins on the OLED were all connected to ground. BS1 has to be tied high to be in I2C mode.
I also found that D2 was shorted to ground as well. The OLED datasheet mentions that D1 and D2 must be tied together to form the SDA line when in I2C mode.
I made these modifications and boy it wasn't pretty. Let's just say a tiny box cutting knife was involved and some edge-of-your-seat soldering to the ribbon breakout.
Finally, I tied the D/C pin to ground, used 1Kohm pullups to VCC from SDA & SCL, used the 0x3C address in code and was then able to talk to the OLED via I2C.
I don't recommend trying what I did, look for the I2C version on the interwebs instead.