I am not seeing the difference in a sea /es decir

Hola Papi

As it says in the lesson, there is no difference, they both mean the same thing. 

The only difference is in register, in this case the level of formality:

• "o sea" is more colloquial (less formal)
• "es decir" is more formal

This is why when you test you will see the hints guide you as to whether the speaker is being more or less formal.

I hope this clarifies things for you.

Saludos 

Shui