[watsisname]

I just hope it goes off in wintertime, when Orion is actually visible from here.

It would be so wrong if the southern hemisphere got that event as well as all the awesome constellations they already have there.

Actually Orion is visible from pretty much anywhere on Earth (arctic circle excluded) at any time of year.  It's not that it's too far south for the summer months, but Earth's orbit brings it near the sun at that time.  Wouldn't matter in the case of it going supernova, though probably from May to July it'd be too close to the sun and lost in the glare.

If you wanna see it at night though (naturally), then yeah, winter would be best when it's near opposite the sun.

[Black Wolf]

I did not mention any O₂ isotopes. By proof of the explosion of a supernova during that period I meant the presence of much heavier elements. We all know that, due to atom degradation, we will never, ever be able to see certain isotopes here on Earth - it doesn't mean that no isotopes of that kind ever existed here, it simply means that all isotopes (again, of that kind) no longer exist due to degradation.

So, if studies prove that a particular kind of isotopes are in rocks originated at the end of the Ordovician, we need to assume that Earth was hit by a flood of elements during that period.

It's pretty much what happened with the meteor impact that might have caused the mass extinction of the Crataceous-Paleocene - research proved how traces of iridium isotopes could be found in rocks that originated during that period.

I know you never mentioned Oxygen isotopes - I thought you might have been getting confused between real phenomena that have been measured at the O-S and something else. As for the rest of it, I think you're getting your physics confused. As has been mentioned, direct gamma interaction would cause elements to lighten, while a flood of heavy elements as direct ejecta from a supernova is extremely unlikely due to the likely distance and the inverse square law. Plus, even if you were talking about direct gamma impact, I can't see how you'd have a measurable change 500 odd million years later. As for the iridium at the K-T boundary, that was a combination (it's assumed) of the residue left after the meteor more or less vaporized on impact and ejecta from the Deccan traps (which were most likely set off by the impact, and so were coincident, stratigraphically. There's no large scale input of matter during a GRB to leave a chemical signature of that nature.
Again though, if you find the original source, please post it, or at least the author and title of the article. I'll probably be able to access it through uni.

Just for the record, I'm not trying to say that a GRB definitely did not cause the O-S extinction. I'm just saying that there's essentially no evidence that it did (only models which say it might have, and no obvious evidence ruling it out), while there's plenty of evidence for tectonic alteration of climate as a cause. As such, while an interesting aside, the GRB theory really doesn't deserve the kind of exposure it's gotten since it was published.