procedure TForm1.Button1Click(Sender: TObject);

type
  PInitDLL = procedure(p: pointer);
var
  hDLLInst: THandle;
  InitDLL : PInitDll;
begin
  hDLLInst :=  0 ;
  AppSession.LogOn;
  if AppSession.Connected then
  try
    hDLLInst:= LoadLibrary('DLL_Doa_Share.DLL');
    if (hDLLInst <=  0 ) then
    raise exception.create('[Неудачный вызов LoadLibrary]');

    @InitDLL := GetProcAddress(hDLLInst,'Init');
    if not assigned(InitDLL) then
    raise exception.Create('[Неудачный вызов GetProcAddress]');

    InitDll(AppSession.ExternalSVC);
  finally
    FreeLibrary(hDLLInst);
  end;
end;

library DLL_Doa_Share;

uses
  SysUtils,
  Classes,
  Unit_DLL in 'Unit_DLL.pas' {Form1};
Exports
   init;

{$R *.RES}

begin
end.

unit Unit_DLL;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  Oracle, StdCtrls;
procedure Init(p : pointer); stdcall;

implementation

procedure Init(p : pointer); stdcall;
var OS : TOracleSession;
begin
  OS := TOracleSession.Create(Application);
  OS.ExternalSVC := p;
 // if OS.Connected then ShowMessage('Connected');
  ShowMessage(OS.ServerVersion);
end;
end.

type
  TMyOracleSession = class(TOracleSession)
end;
.....
OS:TOracleSession;
.....
TMyOrcaleSession(OS).nvhp := ...
TMyOrcaleSession(OS).errhp := ...
.....

     type
  PInitDLL = procedure(p: pointer);

type
  PInitDLL = procedure(p: pointer);stdcall;