I have to admit I'm new to Oxygen.
I'm trying to run the program listed here.

'CLASSES AND METHODS
 '
 
Uses "oxygen","file"
 Dim p0,p1 As Long, src As String
 Dim dd(100) As Double
 
src = "
 
CLASS stat
 METHOD Sum() As Double
 METHOD average() As Double
 /
 da(100) As Double
 dn As Long
 End CLASS
 
'-------------- 
methods of stat
 '==============
 METHOD Sum() As Double
 Dim i
 For i = 1 To THIS.dn
 METHOD += THIS.da(i)
 Next
 End METHOD
 METHOD average() As Double
 METHOD = THIS.Sum / THIS.dn
 End METHOD
 End methods
 '====
 'TEST
 '====
 Dim s As stat
 s.da=>2,4,6,8,10,12,14,16,18
 s.dn=9
 Print `Sum: ` str(s.Sum) ` Average: ` str s.average
 "
 
O2_BASIC src
 
'msgbox 0, o2_view "o2h "+src
 
If Len(O2_ERROR) Then
 MsgBox 0, O2_ERROR : Stop
 End If
 
O2_EXEC
and I get a runtime error.

So I make changes 
'CLASSES AND METHODS
 '
 
Uses "oxygen","file"
 Dim p0,p1 As Long, src As String
 Dim dd(100) As Double
 
src = "
 
CLASS stat
   METHOD Sum() As Double
   METHOD average() As Double
   /
   da(100) As Double
   dn As Long
 End CLASS
 
'-------------- 
methods of stat
 '==============
 METHOD Sum() As Double
   Dim i
   Dim mysum as double        ' Added
   For i = 1 To THIS.dn
'     METHOD += THIS.da(i)
     mysum += THIS.da(i)      ' Added
   Next
   METHOD = mysum             ' Added
 End METHOD
 METHOD average() As Double
    METHOD = THIS.Sum / THIS.dn
 End METHOD
 End methods
 '====
 'TEST
 '====
 Dim s As stat
 s.da=>2,4,6,8,10,12,14,16,18
 s.dn=9
 Print `Sum: ` str(s.Sum) ` Average: ` str s.average
 "
 
O2_BASIC src
 
'msgbox 0, o2_view "o2h "+src
 
If Len(O2_ERROR) Then
 MsgBox 0, O2_ERROR : Stop
 End If
 
O2_EXEC
I make changes, but I still get error.

I have TB 1.8.9.0.

Regards,
Bob