You can find the pin assignment in the hardware design details:
https://github.com/OpenSprinkler/OpenSprinkler-Hardware/tree/master/OS/3.0
OS 3 (based on ESP8266) does NOT use 74HC595, mainly because ESP8266 does not have enough number of GPIO pins. Instead, it uses PCA9555 IO expander, which is I2C-based so only uses 2 pins and are shared with OLED and RTC.