To get the answers you're looking for, you need a couple of figures to start with:
(1) The amount of O2 dissolved in blood plasma: about 0.3 ml O2 per 100 ml plasma;
(2) The amount of O2 bound to hemoglobin in blood: about 20 ml O2 per 100 ml blood. - Ref: https://www.ncbi.nlm.nih.gov/books/NBK54103/ (which also sources the figures used elsewhere here)
Blood makes up about 7% w/v of the human body, so a 70 Kg (154 lb) person will have about 4.9 litres of blood. 5 litres of blood would hold around 1 litre of O2 bound to the hemoglobin. However, around half of the blood will be venous, not arterial, and that only has about 75% O2 saturation, so the actual amount in arterial plus venous blood will be about 1/8 less than that calculation. Obviously, the figure will vary significantly with body weight.
At normal pressure, the arterial hemoglobin is effectively saturated with O2. However, if the ambient pressure increases, then the amount of O2 dissolved in the plasma will increase proportionately (Henry's Law). Similarly, breathing a gas with a greater O2 fraction will increase the amount of dissolved O2 in a linear fashion. Nevertheless, O2's low solubility means that the dissolved O2 is only about 1/60 of that bound to hemoglobin, so unless that is severely compromised (e.g. CO poisoning), increasing the pressure or fraction of O2 has little effect on the supply to the body.
As for the O2 stored in the body, each myoglobin molecule only binds 1 molecule of O2, compared with 4 molecules of O2 per molecule of hemoglobin, so it will contribute a much smaller amount to the total. Of course, that will vary again with body weight but also with the amount of muscle and other type of tissue making up the body.