[Suicide Junkie]

The amount of matter in the universe is just very small compared to its volume.
On the order of 10^-28 kg per cubic meter.
The mass of a proton is 1.67 x 10^-27.

Now, most of the mass is also concentrated in stars, planets and whatnot, so the matter left to spread over deep space is even less.
Interplanetary space has a bit more than deep space, mot not too much.

-----

The oxygen molecules from your tank are all travelling quite fast in random directions, and since there are no longer any walls to hold them in, the random motions cause them to spread out quite quickly. Gravity will deflect them somewhat towards the nearby large bodies, but the velocity of those oxygen molecules is surprisingly high.

After they rapidly leave your immediate vicinity, they'll spread out around the solar system and the concentration drops to undetectable levels again.

A kilogram of oxygen has 2x10^25 molecules, but a cubic AU (radius of earth's orbit) has 3x10^33 cubic meters in it.
Evenly spaced and at orbital speeds, you'd bump into those lost molecules rarely enough to count each meeting.