Build ASCII ladders

Ruby, 71

->m,n{h=0;(?o+?+*(n-1)+?o).chars{|c|puts [?|+' '*m+?|]*h,c+?-*m+c;h=m}}

ungolfed in test program

f=->m,n{
  h=0                             #The number of | above the 1st rung is 0
  (?o+?+*(n-1)+?o).chars{|c|      #Make a string of all the rung ends o++...++o and iterate through it
    puts [?|+' '*m+?|]*h,         #draw h vertical segments |  ...  |
      c+?-*m+c                    #and a rung with the correct ends
    h=m                           #The number of | above all rungs except the 1st is m
  }
}


f[gets.to_i,gets.to_i]

CJam, 43 42 bytes

I'm not sastified by the score. But I'm not Dennis, right?

q~:Z;'-'o{[\Z*1$N]}:X~['-_'+X\'|XZ*]@*1>1$

Input is 2 space separated items. Length first

2 3
o---o
|   |
|   |
|   |
+---+
|   |
|   |
|   |
o---o

Explanation

q~:Z;'-'o{[\Z*1$N]}:X~['-_'+X\'|XZ*]@*1>1$
q~                                         e# read input
  :Z;                                      e# Record the size in Z and discard
     '-'o{[\Z*1$N]}:X~                     e# Create the initial line (and final). also creates a shorcut to do this later
           \                               e# Capture two arguments
            Z*                             e# The separator is repeated size times
              1$                           e# Repeat the first argument
                N                          e# Add newline
                                           e# X is a function to create line in a ladder
                      ['-_'+X\'|XZ*]       e# Design the repeating part
                                    @*     e# Repeat the pattern n times
                                      1>   e# Discard the initial
                                        1$ e# Since the final line is same than the initial, we just write it.
                                           e# Implicit printing

JavaScript (ES6), 89

... repeat, repeat, repeat ...

(n,m,R=x=>x.repeat(m),b=R(`|${R(' ')}|
`),d=`o${c=R('-')}o
`)=>d+R(b+`+${c}+
`,m=n-1)+b+d

Test

F=(n,m,R=x=>x.repeat(m),b=R(`|${R(' ')}|
`),d=`o${c=R('-')}o
`)=>d+R(b+`+${c}+
`,m=n-1)+b+d

// Less golfed
U=(n,m)=>
{
  var R=x=>x.repeat(m),
      a=R(' '),
      b=R(`|${a}|\n`);
      c=R('-'),
      d=`o${c}o\n`;
  m=n-1;
  return d+R(b+`+${c}+\n`)+b+d
}

function test() {
  var i=I.value.match(/\d+/g)
  if (i) O.textContent=F(+i[0],+i[1])
  console.log(i,I.value)
}  
 
test()

N,M: <input id=I value="3,5" oninput=test()>
<pre id=O></pre>