As adjectives the difference between oblong and flat
is that
oblong is longer than wide while
flat is having no variations in height.
As nouns the difference between oblong and flat
is that
oblong is something with an oblong shape while
flat is an area of level ground or
flat can be (archaic|new england|now chiefly british) an apartment.
As an adverb flat is
so as to be flat.
As a verb flat is
(poker slang) to make a flat call; to call without.