  We propose a consistent string theory framework for embedding brane world
scenarios with infinite-volume extra dimensions. In this framework the Standard
Model fields are localized on D3-branes sitting on top of an orientifold
3-plane. The transverse 6-dimensional space is a non-compact orbifold or a more
general conifold. The 4-dimensional gravity on D3-branes is reproduced due to
the 4-dimensional Einstein-Hilbert term induced at the quantum level. The
orientifold 3-plane plays a crucial role, in particular, without it the
D3-brane world-volume theories would be conformal due to the tadpole
cancellation. We point out that in some cases the 4-dimensional Planck scale is
controlled by the size of certain relevant (as opposed to marginal) orbifold
blow-ups. We can then have a scenario with the desirable 4-dimensional Planck
scale, the string scale of order TeV, and the cross-over to 10-dimensional
gravity around the present Hubble size. We discuss some general features as
well as concrete models in this ``Orientiworld'' framework, including those
with D7-branes. We point out that the D7-brane gauge symmetry at the quantum
level becomes part of the 4-dimensional gauge symmetry. We present an N=1
supersymmetric model with 3 chiral generations of quarks and leptons, where the
original gauge group (which contains an SU(6) subgroup) can be Higgsed to
obtain a Pati-Salam model with 3 chiral generations, the Pati-Salam Higgs
fields required to break the gauge group further to that of the Standard Model,
as well as the desired electroweak Higgs doublets. The superpotential in this
model is such that we have precisely one heavy (top-like) generation.
