A few types of bacteria will be jumping with joy. However, there are two sides to the given question:
One: What if the atmospheric Oxygen disappears for 5 Seconds?
Two: What if the molecular Oxygen disappears for 5 Seconds?
Since there are relatively different consequences to both events, we will cover them separately.
21% of our atmosphere is made up of oxygen, atmospheric Oxygen. If it were to disappear for 5 seconds magically, our first guess would be the effect it would cause on our breathing, isn’t it? Well, not really. Our bodies do not identify the lack of Oxygen. However, they do detect a rise in the levels of Carbon Dioxide. The feeling of suffocation only comes from Carbon Dioxide build-up in our blood, and 5-seconds may not be enough to make you feel out of breath. The thing your body will probably notice first is your cells exploding, or the sudden drop in air pressure causing your inner ear to explode. You see, there is a reason that your plane is a pressurized cabin, at higher altitudes, low-pressure air outside the ear drum as opposed to behind the eardrum will cause the ear drum to pop. 21% decrease in air pressure is equal to an elevation of 2000 meters.
Next, the most obvious thing would be that all the fires will go out, which includes the combustion engines in planes and vehicles causing halts for 5 seconds. The near take-off planes will be more damaged and they would crash to the ground due to lack of altitude. For the planes higher in the sky, 5 seconds may provide enough cushion time, before the engines restart again when Oxygen returns. However, it depends on multiple other factors as well that are not in the scope of this discussion.
If you are on the side of the planet where it is daytime, these five seconds are going to be darker. The atmosphere has become 21% lighter, and there are a lesser number of particles to scatter the blue light. So it will be blacker day. Speaking of oceans and daylight, you must avoid being at the beach during this 5-second Oxygen blackout, unless you want to be toasted. The Ozone layer is essentially Oxygen in molecular form, which blocks the ultraviolet light. With Oxygen gone, the ultraviolet lights from the sun will cause severe sunburns.
Untreated metals refuse to fuse into each other during welding due to the presence of an oxidation layer (Oxygen in air). Without this Oxidation layer, the metals will weld together instantly without an intermediate state of liquefaction, the process artificially achieved by Cold welding.
The people on the more urban side of the earth will notice the buildings around them turning to dust. Oxygen is a part of Carbon Dioxide which is an essential binder in concrete structures. Without Carbon Dioxide, the structures will not be able to stay firm and collapse eventually.
Oxygen not only makes up the atmosphere, it also makes up the 45% of the Earth’s Crust and Mantle. Loss of this Oxygen means that the Earth’s Crust will no longer be able to support giant structures, or light structures either. It will cause everything on the Earth to free fall almost instantly.
The molecular Oxygen makes up the water. When the vacuum Oxygen molecules are created, the hydrogen from water molecules will become an unbounded free gas. Hydrogen is the lightest of gasses in the atmosphere, so it will rise above the atmosphere, and will leak into space. Are you worried about the marine life? Well, no need to worry. No living being will witness the evaporation of the ocean, since living cells are also made up of water, and you’ve guessed it! We, along with every living organism, will explode, because our cells are more hydrogen than water. The gaseous Hydrogen will expand in volume and Kaboom!! It is just like being in the space without the spacesuit.
Pretty much everyone will suffer painfully and drastically with our beloved Oxygen nowhere to be seen. There are a few exceptions, hypothetically speaking. There exist single-celled bacteria and protists, a few of them anyway, that consider Oxygen poisonous. These bacteria are linked with lethal illnesses. If you do survive the cell explosions, free falls, sunburns, ear drum explosions, and the collapsing buildings around you, the silver lining is that these bacteria will go through billions of mutations and might even become Oxygen-resistant. Even when the Oxygen returns, they refuse to be eradicated. Congratulations, you have just received a bonus of incurable illness that will stay even when the disaster is over. These were the happy bacteria we mentioned at the start of the discussion.
On the return of Oxygen after 5-seconds, everything will be on fire like Micheal Bay explosions, as the hydrogen present anywhere will react with Oxygen and start a combustion process. In short, the Earth will be extremely cold for those 5-seconds, and then it will explode after 5-seconds.