  We study the interpretation of O7+ planes in F-theory, mainly in the context
of the six-dimensional models. In particular, we study how to assign gauge
algebras and matter content to seven-branes and their intersections, and the
implication of anomaly cancellation in our construction, generalizing earlier
analyses without any O7+ planes. By including O7+ planes we can realize 6d
superconformal field theories hitherto unobtainable in F-theory, such as those
with hypermultiplets in the symmetric representation of su. We also examine a
couple of compact models. These reproduce some famous perturbative models, and
in some cases enhance their gauge symmetries non-perturbatively.
