How do you find oxidation numbers

How do you find the oxidation numbers for the following?:

1. Fe in FeO
2. As in AsO4^3-
3. P in H3PO4
4. C in H2C2O4

I'm not sure how to solve these, any help would be greatly appreciated. :)

We know that H is alsways +1 and oxygen is always -2. Beyond that we know the oxidation numbers of the metals in groups IA and IIA, which are +1 and +2 respectively.

The sum of the oxidation numbers must equal zero for a compound, or the charge on the polyatomic ion.

1. Fe in FeO == +2
2. As in AsO4^3- == +5
3. P in H3PO4 == +5
4. C in H2C2O4 == +3

well there are a few simple rules i'm sure you can find in your notes/book
anyways, it's time for my good deed today

1) oxygen is almost always -II (except in peroxide or in combination with fluorine)
and the sum of every oxidation numbers must be equal to the charge of the molecule
=>Fe +II

2)same rule as above =>As +V

3) another rule, hydrogen is always +I (except in metal hydrides)
P => +V

4) that's a tricky one, you see it depends on which isomer you're looking at
but if we just do it mathematically
=> C +III

Oxidation Number Definition: The oxidation number is the electrical charge that the central atom in a coordination compound would have if all the ligands and electron pairs were removed. Usually the oxidation number has the same value as the oxidation state.
The oxidation number is represented by a Roman numeral. The plus sign is omitted for positive oxidation numbers. The oxidation number is seen as a superscript to the right of an element symbol (e.g., FeIII) or in parentheses after the element name [e.g., Fe(III)] usually with no space between the element name and the parentheses.