How can I run a program in place of an OS ? I mean is it possible for me to replace the OS with a custom made program which says "Hello World" as soon as the Hardware starts ?
Yes it is entirely possible, however this program will have to be your OS. That means that you will not have your typical functions to print to the screen, instead you will either have to implement these functions yourself which write to the hardware or you have to simple write "Hello World" to the graphics hardware directly.
For tutorials on how to do this, look up the OSDev wiki, they have tutorials for this exact sort of thing.
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No. There is too much going on. You need an OS and you replace the existing windows manager/gui/cli/shell with yours. Example, instead of MS Windows "explorer" opening and running you replace it with yours. Not practical in MS but...
You can do this with linux distros... you should check out TinyCore for a really barebones OS that is meant to be built on. Default Windows Manager in TinyCore is flwm which is built on a framework you could use to make your own gui