Are there any other (meta)stable oxygen molecules beyond O3? Apart from Ozone, are there any longer oxygen molecules that are at least stable for short amounts of time? What configurations would that be?
According to my German version of Holleman-Wiberg: Inorganic Chemistry there is one additional, very short lived species, the so called tetraoxygen. A short article is also available on Wikipedia.

Wiberg describes it's synthesis from dioxygen in a mass-spectrometer employing the "neutralisation-reionisation-technique" (I am translating and paraphrasing at the same time, there might be something lost in translation.) It's stable time frame is in the micro second area. The decomposition barrier of $\ce{O4}$ is about 25&nbsp;kJ/mol. The structure is not known, but most likely an aggregate of two oxygen molecules, i.e. $\ce{(O2)2}$. In lower concentrations $\ce{O4}$ might be found in liquid oxygen and they may be a template for the  &epsilon;-high pressure modification of solid oxygen forming at 0.6&nbsp;Mbar.


Holleman, A., Wiberg, N., Wiberg, E., et al. (2008). Lehrbuch der Anorganischen Chemie. Berlin, Boston: De Gruyter. Kapitel XIII 1.1.4 Kurzlebige Sauerstoffspezies. S. 513f.  (in german)
Translation of the 101st edition available as Egon Wiberg and Nils Wiberg (2001) Holleman, Wiberg. Inorganic Chemistry. Academic Press.