I found that omnicppcomplete does not support typedef-ed struct name . I don't know it is a limitation of omnicppcomplet or it is because some missing configuration in my vim enviroment . Let me give you a dummy example to put it more clear.
typedef struct foo {
int a;
int b;
}foo_t ;
foo a ;
foo_t b ;
then after I hit "a." in insert mode , the following will appear in a popup window :
a m + foo
b m + foo
but after hit "b." , the following line appears at the bottom of the sreen :
"Omni completion (^O^N^P) Pattern not found
======================edit after Habi's answer===============================
the ctags version I use is 5.4 and the latest is 5.8 . hint by Habi's example , I download the latest version of ctags and fix the problem .