you dont seem to be fucking up? the absolute simplest way is:
count the number of lines and dots sticking out of the element, then do valance number - number of dots and lines, AKA more lines and dots = more electrons, or more negative charge.
For oxygen its 6, so if you have a single bond and 3 lone pairs, you end up with 3x2= 6 dots and 1 line, so 6-7=-1
..
-O :
..
So, depending on the molecule, its can often be favorable to double bond it, then you lose 2 dots and gain 1 line, for a net loss of 1. Bringing the FC to 0.
..
=O
..
6-6=0
when you have positive central atom, (try doing (SO4)2-, you will end up with a +2 central S and all around you get -1 Oxygen. When that happens, you reduce the number of FC by creating 2 double bonds (S is an exception and can exceed octets), then you get 2 double bonds and 2 single ones, or two negative oxygens only. thats the best structure because O is the most electronegative, you only have 2 formal charges (as opposed to the previous 4), and those 2 charges are very small in absolute value (1)
