Odd looking carbocation stability order I am studying introductory organic chemistry, suddenly this one started looking odd, 

The carbocation stability for three compounds as given by my book is shown below, 



Let me call those as compounds A>B>C as given in image. It is very well known that 'N' and 'O' are electron withdrawing group(shows -I effect) and they can also show +M effect( some texts call it as Resonance effect ) by delocalising their lone pair over the vacant p-orbital of the carbocation. 

Now all i want to know is, How to decide whether +M effect or -I effect will operate in this case? If I consider only inductive effects, the order would be C>A>B   which is wrong in this case. 

One part of my text book says that +M effect > -I effect (in more than 99% of cases). But then too the next question arises, why +M effect of 'N' is more than that of 'O'?

I got stuck in this one and i am now asking whether such questions are feasible without conducting experiments.

Explanation with the principles which are familiar to my level would be nice.
How to decide whether +M effect or -I effect will operate in this case?


The extent of stabilizing effect follows the order: $\ce{\text{Mesomeric} &gt; \text{Hyperconjugation} &gt; \text{Inductive}}$. 

In general, this order is based on extent of $\ce{e-}$ transfer. In mesomeric effect, $\pi$-bonds are in conjugation which completely transfers $\ce{e-}$ density to carbocation. On the other hand, in inductive effect, induction of charge takes place, which partially transfers $\ce{e-}$ density through $\sigma$-bond. Hence, it is weaker.


  why +M effect of 'N' is more than that of 'O'?


As $\ce{N}$ has lower electronegativity than $\ce{O}$, hence it has greater $\ce{e-}$-donating tendency, therefore greater $\ce{+M-\text{effect}}$