The Collaborative International Dictionary of English v.0.48:

Open door \O"pen door\
      (a) Open or free admission to all; hospitable welcome;
          free opportunity.
          [Webster 1913 Suppl.]

                She of the open soul and open door,
                With room about her hearth for all mankind.
                                                  --Lowell.
          [Webster 1913 Suppl.]
      (b) In modern diplomacy, opportunity for political and
          commercial intercourse open to all upon equal terms,
          esp. with reference to a nation whose policy is wholly
          or partially fixed by nations foreign to itself, or to
          territory newly acquired by a conquering nation. In
          this sense, often used adjectively, as, open-door
          system, open-door policy, etc.
          [Webster 1913 Suppl.]

                The steps taken by Britain to maintain the open
                door have so far proved to be perfectly futile.
                                                  --A. R.
                                                  Colquhoun.
          [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

open door
    n 1: the policy of granting equal trade opportunities to all
         countries [syn: open-door policy, open door]
    2: freedom of access; "he maintained an open door for all
       employees"