modifying the reset handler asm code

question 1:
This is the original reset handler that is in the startup file (*.s)

Reset_Handler   PROC
                EXPORT  Reset_Handler             [WEAK]
                IMPORT  SystemInit
                IMPORT  __main
                LDR     R0, =SystemInit
                BLX     R0
                LDR     R0, =__main
                BX      R0
                ENDP

which i modify to

Reset_Handler   PROC
                EXPORT  Reset_Handler             [WEAK]
                IMPORT  SystemInit
                IMPORT  OSInit
                IMPORT  __main
                LDR     R0, =SystemInit
                BLX     R0
                LDR     R0, =OSInit
                BLX     R0
                LDR     R0, =__main
                BX      R0
                ENDP

Here the OSInit is a function. This works fine.

But when i modify the original to following, I get a hard fault.

Reset_Handler   PROC
                EXPORT  Reset_Handler             [WEAK]
                IMPORT  SystemInit
                IMPORT  main
                IMPORT  OSInit
                LDR     R0, =SystemInit
                BLX     R0
                LDR     R0, =main
                BLX     R0                    ;getting hardfault here
                LDR     R0, =OSInit
                BX      R0
                ENDP

Irrespective of whether the OSInit is a function or a task, i get Hardfault.

question 2:
does the '__' (two underscores) before 'main' have any significance. Because when i write __OSInit, the compiler is unable to locate OSInit function (Error L6218E: undefined symbol).
but for '__main' the function name is always 'main' and compiler locates it.