"У Одесі є велика площа."

Translation: There is a big square in Odesa.

[RomanBly1]

Shouldn't Odesa be with two s?
Odessa?

[Mr.Xee]

No, it shouldn't. Ukrainian name of the city has only one S, the same is correct for English name. They have two C only in Russian.

[Freya365]

Yes, traditionally it was spelled 'Odessa', as in the book and film title 'The Odessa File' by Frederick Forsyth.