GA pilot reported filing an IFR flight plan for a flight in IMC. The flight was completed as planned. The pilot realized later; the Obstacle Departure Procedure should have been flown as published.

Narrative

I recently got my instrument rating. I arrived at Astoria Airport on the ILS a couple hours before departure. There was a small scattered layer at 900 feet MSL. At the time of departure; there was now a overcast layer above the airport at the same altitude. I felt very rushed to leave; as the departure was originally planned for XA:30; I filled a near direct IFR flight plan as quickly as possible. I glanced at the ODP (ASTORIA THREE (OBSTACLE) departure) for AST but I did not file the ODP as I believed it would take too long to complete. At the time; I believe that if ATC issues me a clearance than I am safe from hitting something so I thought filing the ODP was unnecessary.I received my clearance and took off shortly after. I climbed on the left downwind Runway 32 blindly through the clouds. I felt very uneasy climbing through the clouds completely blindly but I did what the clearance told me to do which was 'as filed' so I climbed towards the next fix in IMC. I broke out of the clouds and continued on course. There was nothing wrong with the rest of the flight.The next day; still feeling uneasy about the whole situation; I called my CFI. We had a very lengthy conversation about this issue. The next day I talked to other pilots at the Company weekly meeting. I now understand that I am not guaranteed obstruction clearance unless I use the ODP if they are available. So; next time I will be certain to file the ODP if another weather situation like this occurs. I learned that I need to pay close attention the takeoff minimums section of the Terminal Procedures Publication for the departure airport as it contains crucial departure information that I easily missed. I also learned that planning should not be done at the last minute. The stress of leaving late was a contributing factor to this event as well as lack of preflight departure planning. If this event where to repeat itself; I would spend a considerable amount of time in advance planning the departure out of AST in the case of IFR weather and look for potential ways to relax before starting the airplane and taking off.