Calling Oracle stored procedure using Entity Framework with output parameter?

In this case, you shouldn't be calling:

var query = ctx.Database.SqlQuery<CmdRegisterAssetDto>(sql, projectNameParam, countryCodeParam, locationParam, assetRegisteredParam);

But instead be calling:

var result = ctx.Database.ExecuteSqlCommand(sql, projectNameParam, countryCodeParam, locationParam, assetRegisteredParam);

Notice that the only effective difference is that SqlQuery<CmdRegisterAssetDto> was replaced with ExecuteSqlCommand. This also means that the DTO is unnecessary. Otherwise, your code looks like it should work. Here's your original code in its entirety with the changes I mentioned:

string projectName = "EXCO";string location = "ANYWHERE";string countryCode = "XX";using (var ctx = new RAContext()){    var projectNameParam = new OracleParameter("inProjectName", OracleDbType.Varchar2, projectName, ParameterDirection.Input);    var countryCodeParam = new OracleParameter("inCountryCode", OracleDbType.Varchar2, countryCode, ParameterDirection.Input);    var locationParam = new OracleParameter("inLocation", OracleDbType.Varchar2, location, ParameterDirection.Input);    var assetRegisteredParam = new OracleParameter("OutAssetRegistered", OracleDbType.Varchar2, ParameterDirection.Output);    var sql = "BEGIN RA.RA_RegisterAsset(:inProjectName, :inCountryCode, :inLocation, :OutAssetRegistered); END;";    var result = ctx.Database.ExecuteSqlCommand(sql, projectNameParam, countryCodeParam, locationParam, assetRegisteredParam);    assetRegistered = (string)assetRegisteredParam.Value;}

To prove my theory, I reproduced the null behavior that you're experiencing and then made that one change. It hung for a bit (probably to let EF kick into gear), but then executed quickly every time thereafter. In each case, I found a value waiting in the out parameter.

If anyone out there is running into trouble, there's a longhand variation that takes care of the scripting details for you:

string projectName = "EXCO";string location = "ANYWHERE";string countryCode = "XX";using (var ctx = new RAContext())using (var cmd = ctx.Database.Connection.CreateCommand()){    cmd.CommandType = CommandType.StoredProcedure;    cmd.CommandText = "RA.RA_REGISTERASSET";    var projectNameParam = new OracleParameter("inProjectName", OracleDbType.Varchar2, projectName, ParameterDirection.Input);    var countryCodeParam = new OracleParameter("inCountryCode", OracleDbType.Varchar2, countryCode, ParameterDirection.Input);    var locationParam = new OracleParameter("inLocation", OracleDbType.Varchar2, location, ParameterDirection.Input);    var assetRegisteredParam = new OracleParameter("OutAssetRegistered", OracleDbType.Varchar2, ParameterDirection.Output);    cmd.Parameters.AddRange(new[] { projectNameParam, countryCodeParam, locationParam, assetRegisteredParam });    cmd.Connection.Open();    var result = cmd.ExecuteNonQuery();    cmd.Connection.Close();    assetRegistered = (string)assetRegisteredParam.Value;}

As an afterthought, you technically could go with your original solution if you invoked the query immediately after (i.e. query.FirstOrDefault()). The return value of the query would always be null, but your out parameter would at least get populated. This is because EF queries use deferred execution.