How do you know where to draw oxygen in lewis structure?

well, lets see.  I will try to explain this in terms of general rules of thumb......however, if you read more into the theory of lewis structures, you will have your answer.

I would answer this by saying that carbon must always have 4 bonds.  If you already know this (which I'm assuming you do), I would go one step further by saying that oxygen should have two bonds. (so how would it hold both hydrogens whilst being bound to carbon? thats 3 bonds). and you certainly wouldnt have an -OH because then you wouldn't be able to give carbon 4 bonds (unless to double/triple bonded it to a hydrogen...and this is impossible)

short answer - the rules are not changing.  they are:

1) carbon must have 4 bonds
2) oxygen must have 2 bonds
3) hydrogen must have 1 bond