Ghosts and Pumpkins

JavaScript (ES7), 166 164 159 bytes

Saved 5 bytes thanks to Neil

f=(p,g,j=(g*2)**.5+.5|0,G=j>p-1?p?p-1:0:j,P=`
`,i=~j?g-G*++G/2:G,n=i>0?i>g?g:i:0)=>p|g?f(0,g-n,-1,G-1,P+'    ')+P+'((^v^)) '.repeat(p)+'\\{O.O}/ '.repeat(n):''

Formatted and commented

f = (                                    // given:
  p,                                     // - p = number of pumpkins
  g,                                     // - g = number of ghosts
  j = (g * 2) ** .5 + .5 | 0,            // - j = ceil(triangular root of g)
  G = j > p - 1 ? p ? p - 1 : 0 : j,     // - G = max(0, min(p - 1, j))
  P = '\n',                              // - P = padding string (+ line-break)
  i = ~j ?                               // - i =
    g - G * ++G / 2                      //   first iteration: g - G * (G + 1) / 2
  : G,                                   //   next iterations: G
  n = i > 0 ? i > g ? g : i : 0          // - n = max(0, min(i, g)) = number of
) =>                                     //   ghosts to print at this iteration
p | g ?                                  // if there's still something to print:
  f(                                     //   do a recursive call with:
    0,                                   //   - no pumpkin anymore
    g - n,                               //   - the updated number of ghosts
    -1,                                  //   - j = -1 (so that ~j == 0)
    G - 1,                               //   - one less ghost on the pyramid row
    P + '    '                           //   - updated padding string
  ) +                                    //   
  P +                                    //   append padding string
  '((^v^)) '.repeat(p) +                 //   append pumpkins
  '\\{O.O}/ '.repeat(n)                  //   append ghosts
: ''                                     // else: stop

Underlying math

The tricky part is to find out the optimal width G of the ghost pyramid.

The number of ghosts g in such a pyramid is given by:

g = 1 + 2 + 3 + ... + G = G(G + 1) / 2

Reciprocally, the width of a pyramid containing g ghosts is the real root of the resulting quadratic equation:

G² + G - 2g = 0

Δ = 1² - 4(-2g)
Δ = 8g + 1

G = (-1 ± √Δ) / 2

Which leads to the following real root (also known as the triangular root):

G = (√(8g + 1) - 1) / 2

However, the width of the pyramid is also limited by the number of pumpkins: we can have no more than p-1 ghosts over p pumpkins. Hence the final formula used in the code:

j = ⌈(√(8g + 1) - 1) / 2⌉
G = max(0, min(p - 1, j))

ES6 version, 173 171 166 bytes

f=(p,g,j=Math.pow(g*2,.5)+.5|0,G=j>p-1?p?p-1:0:j,P=`
`,i=~j?g-G*++G/2:G,n=i>0?i>g?g:i:0)=>p|g?f(0,g-n,-1,G-1,P+'    ')+P+'((^v^)) '.repeat(p)+'\\{O.O}/ '.repeat(n):''

Test cases (ES6)

f=(p,g,j=Math.pow(g*2,.5)+.5|0,G=j>p-1?p?p-1:0:j,P=`
`,i=~j?g-G*++G/2:G,n=i>0?i>g?g:i:0)=>p|g?f(0,g-n,-1,G-1,P+'    ')+P+'((^v^)) '.repeat(p)+'\\{O.O}/ '.repeat(n):''

console.log(f(0, 1));
console.log(f(1, 0));
console.log(f(1, 1));
console.log(f(2, 1));
console.log(f(2, 2));
console.log(f(3, 1));
console.log(f(3, 2));
console.log(f(3, 3));
console.log(f(0, 4));
console.log(f(3, 0));
console.log(f(7, 6));

Perl, 246 bytes (newlines are not part of the code and are provided solely for readability)

($c,$d)=<>=~/(\d+)/g;
$p="((^v^)) ";$g="\\{O.O}/ ";
for($f[0]=$c;$d>0;$d--){$f[$b+1]+1<$f[$b]?$f[++$b]++:$f[$b]++;$f[0]+=$d,$d=0 if$b==$c-1;$f[$b]==1?$b=0:1}
$h[0]=($p x$c).$g x($f[0]-$c);$h[$_].=$"x(4*$_).$g x$f[$_]for(1..$#f);
say join$/,reverse@h;

Accepts two numbers: pumpkins first, followed by ghosts. Sample input:

5 20

Sample output:

                \{O.O}/ 
            \{O.O}/ \{O.O}/ 
        \{O.O}/ \{O.O}/ \{O.O}/ 
    \{O.O}/ \{O.O}/ \{O.O}/ \{O.O}/ 
((^v^)) ((^v^)) ((^v^)) ((^v^)) ((^v^)) \{O.O}/ \{O.O}/ \{O.O}/ \{O.O}/ \{O.O}/ \{O.O}/ \{O.O}/ \{O.O}/ \{O.O}/ \{O.O}/ \{O.O}/