For the upper opto-isolator in that circuit, when MC_SCL is low, the LED will light and thus cause the phototransistor to conduct. The phototransistor will then pull current through R1 and make SCL low. Similarly with MC_SDA, R2 and SDA_I. When the pins are high, the LED will be off and R1 and R2 will act as pull-up resistors, making the outputs high. That'll make an isolated one-way I2C connection. The centre opto-isolator handles the I2C communication from SDA_O back to MC_SDA.
That part of the circuit looks correct to me, provided the resistors have the correct values.