I had an oral glusoce test done during a clinical trial (in
Australia). My baseline fasting glucose level was 4.4, one hour post
75g oral glucose load, it was 9.6 and at 2 hours it was 3.9. (I know
the reference ranges are different in Aus from the US). I was told
this was normal. But my insulin levels seemed too low.....I will put
in brackets the "normal" reference range supplied by the lab that did
these tests re the insulin. I didn't get the normal range for the
glucose readings).

At baseline, my fasting insulin level was 4 (6-22)
at one hour, insulin was 39 (40-90)
at two hours it was 3 (15-65).

The guy doing the clinical trial (for a weight loss drink) said it was
all OK. I don't have a regular GP as I live on a farm and only did the
trial as I was in the city with my sister when it happened to be
running.

I am overweight - about 90kg female, height 5'4". My cholesterol and
other bloods are very good. My maternal grandmother had age-onset
diabetes.

I know I need to lose weight and I am at higher risk for all sorts of
nasties being obese. I am working on it (I used to be 135kg several
years ago)But I am just curious about what the low insulin levels
actually indicate during that OGTT?
I am 40. Post hysterectomy, still have ovaries. Aside from weight, am
in good health.

As you pointed out, the reference values are not what I'm used to
seeing in the US, but I'll take your word for it that they were
normal.  In regards to the insulin values, it appears that your body
is actually more sensitive to insulin than most people.  This is
probably a good thing, and is actually the opposite of what happens in
Type 2 ('adult onset') diabetes - in diabetes, the body becomes
resistant to the effects of insulin, and must produce more and more to
get the normal physiological effect.  Your body, however, appears to
be functioning on less insulin.  So, as long as your blood sugars are
normal, these values should not be a problem at all.