Converting Disolved Oxygen from micromole/L to millimole/kg
Oxygen is barely soluble in water, so honestly, treating the saltwater as though it were "pure" saltwater with no dissolved oxygen is roughly equivalent to treating it as heavily oxygenated.  The difference is in the fourth or fifth decimal place.  Note that this would start to fall apart if you are concerned about the solubility of high pressure oxygen in water, but for small quantities of oxygen (such as the micromoles and millimoles you're using) this is likely not a factor.

The conversion is relatively straightforward.  For a solution with concentration $X$:

$$(X\ \mathrm{\mu mol/L}) \times (1\ \mathrm{mmol}/1000\ \mathrm{\mu mol})\times(1\ \mathrm L/1.027\ \mathrm{kg}) = 0.0009737\ X\ \mathrm{mmol/kg}$$