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I'm intend to use coff for my binaries.
I get the coff data structures from Delorie.
It will be a copyright problem if I use them in a header file?

If you copy and paste the code then you are subject to the DJGPP licence which is probably GPL as GCC is. If your code is GPL then there isn't a problem, if it isn't then you'll have to create your own implementation.

From DJGPP/includes/coff.h:

/* Copyright (C) 2003 DJ Delorie, see COPYING.DJ for details */

From DJGPP/copying.dj:

Source code copyright DJ Delorie is distributed under the terms of the GNU General Public Licence, with the following exceptions:

• Sources used to build crt0.o, gcrt0.o, libc.a, libdbg.a, and libemu.a are distributed under the terms of the GNU Library General Public License, rather than the GNU GPL.
• Any existing copyright or authorship information in any given source file must remain intact. If you modify a source file, a notice to that effect must be added to the authorship information in the source file.
• Runtime binaries, as provided by DJ in DJGPP, may be distributed without sources ONLY if the recipient is given sufficient information to obtain a copy of djgpp themselves. This primarily applies to go32-v2.exe, emu387.dxe, and stubedit.exe.
• Runtime objects and libraries, as provided by DJ in DJGPP, when linked into an application, may be distributed without sources ONLY if the recipient is given sufficient information to obtain a copy of djgpp themselves. This primarily applies to crt0.o and libc.a.

That means you're basically stuck with the GPL - which is great if you want your OS to be GPL, which is s****y if you don't.

You can, of course, take the information on the COFF structures from the DJGPP sources, and implement your own includes. You stand a fair chance of not getting into troubles that way, but me not being a lawyer I won't give you any guarantees on that.