When we want to introduce an extra explanation, clarification or a consequence of something that has just been mentioned previously, we can use the expression "o sea" or "es decir" then add the explanation/clarification/consequence.
Using one or the other will depend on the formality or informality of the sentence and the context.
O sea = more colloquial
Mañana es lunes, o sea, tengo que volver a la rutina.Tomorrow is Monday, in other words, I have to go back to my routine.
Al final he salido pronto del trabajo, o sea, que tengo tiempo para una cerveza.In the end I left work early, so, I have time for a beer.
He visto a Carlos con otra chica, o sea, que me está siendo infiel.I saw Carlos with another girl, that is, he is being unfaithful.
Es decir = more formal
Estudiaremos las civilizaciones precolombinas, es decir, antes de la llegada de Colón.We'll study the prehispanic civilisations, i.e, before the arrival of Columbus.
Deben firmar en las dos páginas, es decir, en la página 2 y la página 4.You must sign on both pages, i.e, on page 2 and page 4.
Este paquete lleva incluida la media pensión, es decir, el desayuno y la cena.This package includes half board, that is, breakfast and dinner.
Both o sea and es decir are invariable expressions, i.e they do not change. It is also important to note that they must always be written as two words, not joined together.
This is incorrect:
Note that you may sometimes see o sea at the beginning of a sentence, not being used with the function described above.
In this case it is set expression in colloquial Spanish, used to start a sentence with no specific meaning. It is the equivalent to the word "So..." in English, when starting a sentence.
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