>This procedure is just for illustration purposes.
>

>CREATE PROCEDURE usp_test
>	@Number	INT
>AS
>RETURN @Number + 3
>GO
>

>
>I call it this way
>

>LOCAL x AS Integer, y AS Integer
>x = 5
>?oData.oConn.Execute("{y = call usp_test(x)}")
>

>and I get SQL error "Syntax error or access ciolation."

Where did I say use x,y in {} syntax? OK maybe I wasn't clear. Here is a series of ways to do it.
First for the below sample this is stored procedure (create in Northwind database):

CREATE PROCEDURE usp_test 
 @country varchar(10), @customers int output AS 
 select * from customers where country = @country
 set @customers = @@rowcount
 if ( @customers > 10 )
    return 33
 else
    return 22

Thanks to you BTW. Rechecking this ADO thing I found the piece I was missing (it was in fine print - to get the return value recordset needs to be closed)

#DEFINE adInteger	3
#DEFINE adCurrency	6
#DEFINE adDate		7
#DEFINE adBoolean	11
#DEFINE adChar		129
#DEFINE adNumeric	131
#DEFINE adVarChar	200
#DEFINE adDBTimeStamp	135
#DEFINE adParamInput	1
#DEFINE adParamOutput	2
#define adParamReturnValue 4
#DEFINE adCmdStoredProc	4

clear
Local cn As ADODB.Connection,;
  cmd As ADODB.Command,;
  rs As ADODB.Recordset
cn  = CreateObject('ADODB.Connection')
cmd = CreateObject('ADODB.Command')
rs  = CreateObject('ADODB.Recordset')

cn.ConnectionString = "Provider = SQLOLEDB;"+;
 "Server=servername;Database=Northwind;Trusted_Connection=yes"
cn.Open

cmd.ActiveConnection = cn

* Way 1
cmd.CommandText = "usp_test"
cmd.CommandType = 4 && adCmdStoredProc

* Set up parameters
retParm = cmd.CreateParameter("Return", adInteger, adParamReturnValue)
inParm = cmd.CreateParameter("Input", adVarChar, adParamInput,10)
outParm = cmd.CreateParameter("Output", adInteger, adParamOutput)

cmd.Parameters.Append(retParm)
cmd.Parameters.Append(inParm)
cmd.Parameters.Append(outParm)
  
lcCountry = Trim(InputBox("Enter country:"))
inParm.Value = m.lcCountry

* Way 2
*!*	cmd.CommandText = "usp_test"
*!*	cmd.CommandType = 4 && adCmdStoredProc

*!*	* Set up parameters asking backend
*!*	cmd.Parameters.Refresh
*!*	  
*!*	lcCountry = Trim(InputBox("Enter country:"))
*!*	cmd.Parameters("@country").Value = m.lcCountry

* Way 3
*!*	cmd.CommandText = "{? = Call usp_Test(?,?)}"
*!*	cmd.CommandType = 1 && text

*!*	* Set up parameters
*!*	retParm = cmd.CreateParameter("Return", adInteger, adParamReturnValue)
*!*	inParm  = cmd.CreateParameter("Input", adVarChar, adParamInput,10)
*!*	outParm = cmd.CreateParameter("Output", adInteger, adParamOutput)

*!*	cmd.Parameters.Append(retParm)
*!*	cmd.Parameters.Append(inParm)
*!*	cmd.Parameters.Append(outParm)
*!*	  
*!*	lcCountry = Trim(InputBox("Enter country:"))
*!*	inParm.Value = m.lcCountry

* Execute command
rs = cmd.Execute
* Show result
DO While !rs.EOF
    ? rs.Fields(0).Value
    rs.MoveNext
enddo

* Need to close recordset before getting return 
* and output parameters.
rs.Close

* check return values - 1 output and 1 return code
* For Way 1 and Way 3
? "Return code: ", Cmd.Parameters("Return").Value && or Parameters(0)
? "Total rows: ", Cmd.Parameters("OutPut").Value && or Parameters(2)

* For Way 2
*!*	? "Return code: ", Cmd.Parameters(0).Value && or Parameters(0)
*!*	? "Total rows: ", Cmd.Parameters("@customers").Value && or Parameters(2)

cn.Close

Please note that return value is always the first parameter -Parameters(0)-.
In way 1 and 3 parameters append order exactly matches SPs definition (positional) - Parameters(0)=return value, Parameters(1) = first parameter, Parameters(2) = second parameter and so on. Name of the parameters has no importance.
In way 2 instead of appending parameters we 'ask' it to the stored procedure (note that this would cause a trip to server) and use named parameters (names match to that of SP parameter names) OR you might use ordinals - Parameters(0),Parameters(1)...
Hope it's a little more clear now.
PS:With a need to close the RS to get reurn and output parameters you might consider not to use them depending on your needs.
Cetin