You can write to the National Archives at:
National Archives and Records Administration
Modern Military Reference Branch
8601 Adelphi Road
College Park, MD 20704
Ask them for a copy of the Movement Card for the Richard V Oulahan. This will show you all the places the ship went, usually showing date and place of departure and date and place of arrival.
The movement card will indicate (sometimes) what convoy, if any the ship was part of. You can find the voyage details for the trip just prior to the voyage where she was sunk, and ask the Archives for a copy of the Armed Guard Voyage Report for that voyage (either tell them the convoy or the dates). The voyage report usually details events of the trip, and also frequently includes the crew list (both Armed Guard and Merchant seamen).
You should specifically ask for the crew list when asking for the voyage report. This will tell you who was on board prior to the last voyage, but that is no guarantee that the same people were on board this voyage.
The Archives sometimes (usually these days) charges for the material. If you initially ask for just the movement card, you'll probably get that for free (I have been successful with that), but if the other material is more than a few pages they usually send you an estimate of the cost. Don't be surprised if it takes weeks to get a response. Sometimes I get replies in a few days, other times it takes many weeks.