Quick answer
o sea
Natural: I mean / in other words / so
Literal: “or it be”
How it is used
O sea, no me interesa.I mean, I’m not interested.
Llego tarde, o sea, sobre las nueve.I’ll be late, that is, around nine.
O sea que no vienes.So you’re not coming, then.
Es barato, o sea, para lo que ofrece.It’s cheap — for what it offers, I mean.
Where it is said
Used across the whole Spanish-speaking world. In Spain and Mexico it is also a filler in fast speech, much like English “like”, and heavy use marks a speaker as young or casual.
A mini dialogue
— ¿Entonces lo dejas?— So you’re quitting?
— O sea, sí, pero no ahora mismo.— I mean, yes, just not right now.
Watch out
o sea
It is two words and sea never changes: o sean does not exist.
o sea
When a full clause follows, add que: o sea que no vienes, not o sea no vienes.
Check yourself
What does “o sea” do in a sentence?