Question: How to determine the order of nucleophilicity for given chemical species?

Like I came across this question, to rearrange $\ce{RCOO-, OR-, OH-, H2O}$ (alkyl acetate, alkoxide, hydroxide, and water) in decreasing order of nucleophilicity and it ranked $\ce{OR-}$ first.

Shouldn't it be $\ce{OH-}$ since $\ce{OR-}$ would be more sterically crowded?

Answer: 
In this case you can look at the pKa values of their conjugate acids. Higher pKa of conjugate acid means a weaker acid hence a stronger base. The pKa of water is 15.7, while most alcohols have pKa of 16-18. 

I guess this question is asking for a general trend. Although alkoxide could be sterically hindered, for which an extreme case would be a tertiary alkoxide, but if the electrophile is not hindered at all (e.g. primary alkyl halide), then OR⁻ is still stronger than OH⁻.
OR¯ is NOT RO¯ !!!
Clearly, R¯ is an unknown species and nothing else can be said about it.
OR¯ would be an unusual structure, given oxygen's electronegativity compared to carbon's. In the case of RO¯, the R "group" would have to be very large and bulky to dramatically sterically affect (hide) the O¯ from other molecules. So for simple organic compounds, the effect will be quite small. As already said pKa of the conjugate acid would be an excellent guide to nucleophilicity (in the relevant solvent).