die Oberseite
top side
The upper surface or side of an object, as opposed to the underside.
Die Oberseite des Blattes ist dunkelgrün und glänzend.
The top side of the leaf is dark green and shiny.

top side
Loading your saved state.
top side
The upper surface or side of an object, as opposed to the underside.
Die Oberseite des Blattes ist dunkelgrün und glänzend.
The top side of the leaf is dark green and shiny.

The word is clearly made up of 'ober,' a form of 'oben' (above/on top), and 'Seite' (side). So it literally describes the upper side of something.
Bitte wische die Oberseite des Tisches ab.
Please wipe the top side of the table.
To say what a top side belongs to, you usually use the genitive or a 'von' construction. The genitive form often sounds somewhat more formal.
Die Oberseite des Buches war mit Staub bedeckt.
The top side of the book was covered with dust.
The word is often used for flat objects. Typical examples are the top side of a table, a leaf, a roof, or a hand.
Die Oberseite seiner Hand war von der Sonne verbrannt.
The back of his hand was sunburned.
area / surface
'die Fläche' and 'die Oberseite' can both refer to an area on an object. But 'die Fläche' is more general: it can be a flat area, for example on a table, on a wall, or on the floor. Often it is also about how large this area is. 'die Oberseite' more specifically means the top side of an object, as opposed to the underside. The words can be confused when talking about the upper surface of something. If the position on top is important, 'Oberseite' fits better. If it is generally about the area or the size, 'Fläche' is more natural.

Auf der Oberseite des Kartons steht die Adresse.
The address is written on the top side of the box.
Die Fläche auf dem Tisch ist groß genug für den neuen Drucker.
The surface of the table is large enough for the new printer.