I was quite upset to learn of the recent problem in the US where a bus
carrying evacuees from a medical establishment was involved in some
kind of explosion causing associated deaths.
I did later learn that there were a number of oxygen sources possibly
involved.

This leads to a circumstance that most people especially with breathing
problems that need supplementry oxygen would not be aware of and does
need elaboration.

There are numerous sources where oxygen may be gotten or concentrated
which most people would be aware of, these might be simple chemical
generators which produce oxygen suitable for short term use (common on
aircraft, often mounted inside seat backs or ceilings and fired by a
lanyard)
and of course bottled oxygen which is common everywhere usually with a
common screw type valve.
There are colour coding standards for these bottles which determine
there intended useage.
This should not be confused with bottled air or the type of air in
SCUBA type underwater tanks which is normally only simple compressed
air.

As everyone is aware normal atmospheric air is 20% oxygen and 80%
nitrogen, there are some other gases usually present but not worthy of
mention because of the low concentration.

Things that burn in a normal atmosphere burn more readily as the oxygen
content increases.
If we reach an atmosphere of 100% oxygen, things drastically change.
The spontaneous combustion point of most substances capable of burning
is commonly well within that envelope.
This especially applies to petroleum kinds of substances which commonly
can reach explosive properties.
Fabrics treated with some combustible chemicals also become a
consideration.
If you want to make a really good bomb all you need is a forty-four
gallon drum of kerosene with an oxygen bottle in it and some remote
means of opening the tap, this is probably better than fertiliser.

To begin with, anytime a cylinder is opened it should be done slowly so
as not to allow any sudden pressure circumstances to happen. Any
equipment used must be completely clear of any substance especially in
the petrochemical kinds of area.
Cleaning of equipment and masks must be done only with approved
disinfection materials which specifically are designed for this
purpose. Do not use ordinary disinfectants.
Anytime oxygen is being used it should be understood that a well
venitlated area is neccessary and that the local area must never
become excessively oxygenated.
Any tight control valve must never be lubricated, replace it.
Anytime 100% oxygen is being used, especially with a full face mask it
must be understood that substances on the person's skin may cause
sudden explosion and burning. This has happened with the most odd
substances, one chap had a waxed mustache which suddenly burst into
flames and there have been cases of women with facial toiletries also
having similar problems.

Other typical kinds of situations have happened, one where a dive
instructor was working in a large dive boat below deck cleaning down
the engines with solvent, he normally blew the residue off with
compressed air or one of the half empty dive cylinders, he used a
portable oxygen cylinder assuming it was air.
The explosion did not immediately happen and the air was filled with
atomised solvent and the work partially completed before the learning
experience began. The actual deck lifted from the hull, the contained
fiery conflagration burned him badly but he did manage to get out with
something like 90% burns. The boat was a wreck.

I do not have any information on the US evacuation bus event but am
anxious to learn what exactly happened. Quite obviously the deaths were
caused by burns from the explosion but the actual explosive fuel is my
interest.
Does anyone have any idea of the actual circumstances.

Anyone using oxygen from a cylinder should never allow the thing to run
low or show a contained pressure less than 100 PSI or similar. The
reason for this is to ensure that no external atmospheric air can enter
the bottle and contaminate it's interior.
There are various bugs that can proliferate in a high oxygen
environment and if a flattened cylinder is refilled without a special
purging process, it can (over a period) produce a delightful aromatic
breathing experience.
Something similar to breathing vomit!!

Breathing quality oxygen has higher specification than industrial
oxygen, the main criterea is lower moisture content.

If you ever use a cylinder, especially a portable type, never wrap it
in a blanket or any item where and leakage may proliferate, and never
place the bottle near any fuel type substance.

There was a post on this site previously where portable bottles were
discussed as being used or left in a motor vehicle.
Consider this, a bottle is used at some rate of flow and when the
person reaches his destination he quickly turns the thing off and
places it on the seat. So if this vehicle were parked in the sun for
any period the in-vehicle temperature could be in the area of 60degs C,
if there were any possibility of the bottle not being entirely closed
and some leakage were happening, possibly with a mask attached or
whatever, that the owner would probably have faced it toward the
vehicle seat to prevent any stuff from within the local atmosphere
getting into the mask, the seat of the vehicle of course would most
likely be some kind of velour or fabric material, most likely
containing some scotchguard kind of chemical.
Would it be reasonable to expect that his may be a dangerous kind of
situation in a contained space understanding the possibility of
spontaneous combustion happening in a localised area on the seat?
If a fire were to suddenly happen in this unattended vehicle on the
seat, is it conceivable that the generated heat might cause the nearby
bottle overpressure disk to rupture and imediately expel the entire
pressurised contents of this bottle within the cab of this closed
vehicle?
Of course it is, and it is also most likely that some form of explosion
would also be involved that would most likely send glass particles and
shrapnel outward from the vehicle, probably wouldn't do the vehicle
much good either!
Consider it, and never leave any oxygen bottle in any closed vehicle
unless it is in proper stowage and can not be overheated in any way.
Never leave a bottle in the sun and always consider the possibility of
what might happen if an overpressure disk might rupture with regard to
items nearby and fuel sources.

So what might have happened in the bus?

Cheers, Merlin