I have been a painter of Gamesworkshop models for a number of years and do not really have experience of OO figures but some basic principles for models are
start with undercoat black (yeah ok that's obvious) but this will leave you with creases showing when you paint clothes etc.
dry brushing is one of the best techniques going for models, using acrylic paint, draw out most of the paint from the brush until is almost dry, draw the brush across the area you want to paint and it will leave traces of paint, keep going until you get the depth of colour required.
for flesh colours use a base of red or light brown it sounds daft but as you apply lighter colours particularly white shades they should take a tinge of the base coat.
the best way I found to paint eyes was not to do them like a dot but more like a cat I.e white pupil and a black line down.
a coating of ink can bring out great detail
please remember I was painting models for war gaming and so the need was for certain races to look aggressive and others to look human these were always the hardest
I hope any of this might be help