  We propose 5d descriptions of 6d ${\cal N}=(1,0)$ superconformal field
theories arising from Type IIA brane configurations with an $O8^-$-plane. We
T-dualize the brane diagram along a compactification circle and obtain a
5-brane web diagram with two $O7^-$-planes. The gauge theory description of the
resulting 5d theory for a given 6d superconformal field theory is not unique,
and we argue that the non-uniqueness leads to various dual 5d gauge theories.
There are three sources which lead to the 5d dualities. One type comes from
either resolving both or one of the two $O7^-$-planes. The two situations give
us two different ways to read off a 5d gauge theory from essentially the same
web diagram. The second type originates from different distributions of D5 or
D7-branes, shifting the gauge group ranks of the 5d quiver theory. The last one
comes from the 90 or 45 degree rotations of the 5-brane web diagram, which is a
part of the $SL(2,\mathbb{Z})$ duality of Type IIB string theory, leading to
completely different group structure. These lead to a very rich class of
dualities between 5d gauge theories whose UV completion is the same 6d
superconformal field theory. We also explore Higgsing of the 6d theories and
their 5d counterparts. Decoupling the same flavors from the dual 5d theories
gives rise to another dual 5d theories whose UV completion is the same 5d
superconformal field theory. Finally we propose the 6d description of 5d
theories which is obtained by a generalization of 5d $T_N$ theories with
additional flavors, which turns out not to be in the class of Type IIA brane
construction generically.
