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It can indeed get quite confusing.

For the HX and Fan, this is actually exactly the same way you would do with a regular AirLoopHVAC: the HX and relief fan are going to be specified in relation to the AirLoopHVAC:OutdoorAirSystem. The weird thing is that your DOAS Coils and Fans are ALSO going to be specified in relation to the AirLoopHVAC:OutdoorAirSystem.

Another thing is that you won't have an OutdoorAir:Mixer (which makes it clearer which node is which node...).

Anyways, after using the right node connections, I can get a simulation to run properly (with some warnings, but I'm not trying to do something production ready here). Node connections is really the tricky bit, but OpenStudio SDK can help you do that, which is what I've done.

The other thing is is the order of the equipment on AirLoopHVAC:OutdoorAirSystem:EquipmentList matters a lot (the warning says so!). I chose to put the supply Fan first in line, and the relief last in line.

I have saved a working IDF file for you in this gist: https://gist.github.com/jmarrec/3dc370ccde2e5e43b8b9811627911a1f . Search for "DOAS" (around line 1450), as I have been careful to name things properly.

The TL;DR is this:

AirLoopHVAC:DedicatedOutdoorAirSystem,
  AirLoopDOAS,                            !- Name
  AirLoopDOAS OA Sys,                     !- AirLoopHVAC:OutdoorAirSystem Name
  Always On Discrete,                     !- Availability Schedule Name
  AirLoopDOAS Mixer,                      !- AirLoopHVAC:Mixer Name
  AirLoopDOAS Splitter,                   !- AirLoopHVAC:Splitter Name
  4.5,                                    !- Preheat Design Temperature {C}
  0.004,                                  !- Preheat Design Humidity Ratio {kgWater/kgDryAir}
  17.5,                                   !- Precool Design Temperature {C}
  0.012,                                  !- Precool Design Humidity Ratio {kgWater/kgDryAir}
  2,                                      !- Number of AirLoopHVAC
  LOOP1,                                  !- AirLoopHVAC Name 1
  LOOP2;                                  !- AirLoopHVAC Name 2

AirLoopHVAC:OutdoorAirSystem,
  AirLoopDOAS OA Sys,                     !- Name
  ,                                       !- Controller List Name
  AirLoopDOAS OA Sys Equipment List;      !- Outdoor Air Equipment List Name

OutdoorAir:NodeList,
  AirLoopDOAS OA Sys OA Inlet Node;       !- Node or NodeList Name 1

! ORDER MATTERS HERE
AirLoopHVAC:OutdoorAirSystem:EquipmentList,
  AirLoopDOAS OA Sys Equipment List,      !- Name
  Fan:SystemModel,                        !- Component Object Type 1
  AirLoopDOAS Supply Fan,                 !- Component Name 1
  HeatExchanger:AirToAir:SensibleAndLatent, !- Component Object Type 2
  AirLoopDOAS HX,                         !- Component Name 2
  CoilSystem:Heating:DX,                  !- Component Object Type 3
  AirLoopDOAS HC CoilSystem,              !- Component Name 3
  CoilSystem:Cooling:DX,                  !- Component Object Type 4
  AirLoopDOAS CC CoilSystem,              !- Component Name 4
  Fan:SystemModel,                        !- Component Object Type 5
  AirLoopDOAS Relief Fan;                 !- Component Name 5

HeatExchanger:AirToAir:SensibleAndLatent,
  AirLoopDOAS HX,                         !- Name
  [...]
  AirLoopDOAS OA Sys OA Inlet Node,       !- Supply Air Inlet Node Name
  AirLoopDOAS OA Sys HX Outlet Node,      !- Supply Air Outlet Node Name
  AirLoopDOAS OA Sys Return Air Node,     !- Exhaust Air Inlet Node Name
  AirLoopDOAS OA Sys HX Relief Outlet Air Node, !- Exhaust Air Outlet Node Name
  [...]
  Yes;                                    !- Economizer Lockout

CoilSystem:Heating:DX,
  AirLoopDOAS HC CoilSystem,              !- Name
  Always On Discrete,                     !- Availability Schedule Name
  Coil:Heating:DX:SingleSpeed,            !- Heating Coil Object Type
  AirLoopDOAS HC;                         !- Heating Coil Name

Coil:Heating:DX:SingleSpeed,
  AirLoopDOAS HC,                         !- Name
  [...]
  AirLoopDOAS OA Sys HX Outlet Node,      !- Air Inlet Node Name
  AirLoopDOAS HC Outlet to CC Inlet Node, !- Air Outlet Node Name
  [...]
  2000;                                   !- Resistive Defrost Heater Capacity {W}

CoilSystem:Cooling:DX,
  AirLoopDOAS CC CoilSystem,              !- Name
  Always On Discrete,                     !- Availability Schedule Name
  AirLoopDOAS HC Outlet to CC Inlet Node, !- DX Cooling Coil System Inlet Node Name
  AirLoopDOAS CC Outlet Node,             !- DX Cooling Coil System Outlet Node Name
  AirLoopDOAS CC Outlet Node,             !- DX Cooling Coil System Sensor Node Name
  Coil:Cooling:DX:SingleSpeed,            !- Cooling Coil Object Type
  AirLoopDOAS CC;                         !- Cooling Coil Name

Coil:Cooling:DX:SingleSpeed,
  AirLoopDOAS CC,                         !- Name
  [...]
  AirLoopDOAS HC Outlet to CC Inlet Node, !- Air Inlet Node Name
  AirLoopDOAS CC Outlet Node,             !- Air Outlet Node Name
  [...]
  10;                                     !- Basin Heater Setpoint Temperature {C}

Fan:SystemModel,
  AirLoopDOAS Supply Fan,                 !- Name
  [...]
  AirLoopDOAS CC Outlet Node,             !- Air Inlet Node Name
  AirLoopDOAS Supply Fan Outlet Node,  !- Air Outlet Node Name
  [...]
  1;                                      !- Number of Speeds

Fan:SystemModel,
  AirLoopDOAS Relief Fan,                 !- Name
  Always On Discrete,                     !- Availability Schedule Name
  AirLoopDOAS OA Sys HX Relief Outlet Air Node, !- Air Inlet Node Name
  AirLoopDOAS OA Sys OA Relief Node,      !- Air Outlet Node Name
  [...]
  1;                                      !- Number of Speeds

AirLoopHVAC:Mixer,
  AirLoopDOAS Mixer,                      !- Name
  AirLoopDOAS OA Sys Return Air Node,     !- Outlet Node Name
  LOOP1 Exhaust Node,                     !- Inlet Node Name 1
  LOOP2 Exhaust Node;                     !- Inlet Node Name 2

AirLoopHVAC:Splitter,
  AirLoopDOAS Splitter,                   !- Name
  AirLoopDOAS Supply Fan Outlet Node,             !- Inlet Node Name
  LOOP1 OA Inlet Node,                    !- Outlet Node Name 1
  LOOP2 OA Inlet Node;                    !- Outlet Node Name 2