the example OxygenBasic\examples\Parameters\DefaultByval1.o2bas needs tidying.
edit, I completely missed the #byval and #byref statements, somehow I thought that they were comments.
#byval
sub f1(a as long) as long
'=======================
int a=4
end function
int a=2
f1 a
print a
#byref
sub f2(a as long) as long
'=======================
int a=4
end function
int a=2
f2 a
print a