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"

Moby Thesaurus II by Grady Ward, 1.0:

65 Moby Thesaurus words for "open door":
   Eisenhower Doctrine, Monroe Doctrine, Nixon Doctrine,
   Truman Doctrine, access, amiability, appeasement, balance of power,
   bonhomie, brinkmanship, coexistence, colonialism, compromise,
   containment, cordiality, detente, deterrence, diplomacy,
   diplomatic, diplomatics, dollar diplomacy, dollar imperialism,
   entrance, entree, entry, expansionism, foreign affairs,
   foreign policy, friendliness, generosity, geniality,
   good-neighbor policy, graciousness, heartiness, hospitableness,
   hospitality, imperialism, in, internationalism, isolationism,
   liberality, manifest destiny, militarism, nationalism,
   neighborliness, neocolonialism, neutralism, nonresistance,
   open arms, open-door policy, openheartedness, opening,
   peace offensive, peaceful coexistence, preparedness, receptiveness,
   shirt-sleeve diplomacy, shuttle diplomacy, spheres of influence,
   the big stick, tough policy, warmheartedness, warmness, warmth,
   world politics