In this article we conjecture a relationship between 5d SCFT's, that can be
engineered by 5-brane webs in the presence of an O5β-plane, and 4d class S
theories of type D. The specific relation is that compactification on a circle
of the former leads to the latter. We present evidence for this conjecture. One
piece of evidence, which is also an interesting application of this, is that it
suggests identifications between different class S theories. This can in turn
be tested by comparing their central charges.Comment: 30 pages, 18 figures; v2 minor revision, version published in JHE