How real do you want this?
Real enough that it could actually happen (why? — are you hoping to do it?)
or just real enough to be plausible in a work of fiction?
As you probably know, diamonds and graphite (pencil “lead”)
are both pure carbon.
They are distinguished by different crystal structures.
They are called allotropes, and there are others.
Warning: spoilers ahead.
Polymorphs are similar to allotropes,
but the term allotrope applies only to elements,
while polymorph applies only to compounds.
Polymorphs are most often found in minerals and organic compounds
(e.g., pharmaceuticals).
There are polymorphs of ice (solid $\mathrm{H_2O}$),
but the ordinary ice that forms on puddles in winter
(and that we make in our freezers and put into our drinks)
is the only one that can exist at standard pressure.
However, Kurt Vonnegut’s novel Cat’s Cradle stipulates
that there are others that can exist at standard pressure.
In particular, one called “ice-nine”
has a melting point of $\mathrm{45.8\:°C}$ $(\mathrm{114.4\:°F})$,
so it is stable at room temperature — and, indeed, on most of the Earth.
It has the additional property
that if any liquid (or gaseous) $\mathrm{H_2O}$
comes into contact with ice-nine,
it immediately freezes and also becomes ice-nine.
For most of the book,
the ice-nine is kept carefully controlled and locked away.
But eventually it escapes into the environment
and causes all the oceans to freeze,
and it’s pretty much curtains for mankind.
Oxygen has allotropes, too.
The best known ones are ordinary, atmospheric oxygen $(\mathrm{O_2})$
and ozone $(\mathrm{O_3})$,
but there are others, including tetraoxygen $(\mathrm{O_4})$,
which is unstable (or metastable).
Wikipedia notes:
Triatomic oxygen (Ozone, O3),
is a very reactive allotrope of oxygen
that is destructive to materials like rubber and fabrics
and is also damaging to lung tissue.
Traces of it can be detected as a sharp, chlorine-like smell,
coming from electric motors, …
Now suppose that $\mathrm{O_5}$ is stable
at standard atmospheric pressure and temperature.
And suppose that, like ice-nine, it acts as a catalyst,
converting $\mathrm{O_2}$ (and maybe also $\mathrm{O_3}$) into $\mathrm{O_5}$,
and that this conversion is very hard to undo (think particle accelerator).
It would be quite reasonable to expect $\mathrm{O_5}$
not to react chemically the way $\mathrm{O_2}$ does
(remember the differences
between the characteristics of diamonds and graphite).
Once this stuff got into the lungs of an oxygen breather,
it would convert all the ordinary oxygen
(including, eventually, the stuff in hemoglobin) into $\mathrm{O_5}$,
which would not give the cells what they need.
You’d suffocate.
Or maybe it’s just very, very bad for you — like ozone.
I don’t know how $\mathrm{O_5}$ would be created with pre-WWI technology,
but the existence of tetraoxygen $(\mathrm{O_4})$
was first predicted in 1924 as a result of experiments with liquid oxygen —
so that technology existed then.
You would need very little handwavium to explain how somebody cooled oxygen,
past the point where it condenses into a liquid,
near to its freezing temperature,
and this caused $\mathrm{O_5}$ crystals to form.
When allowed to thaw, they did not decompose,
but rather started a Cat’s Cradle-like chain reaction
that converted all the oxygen on the planet into toxic $\mathrm{O_5}$.