The U.S. Constitution is the foundation of all U.S. law.

The branches of the U. S. government are established by U.S. Constitution.

The Legislative branch is established by Article I of the U. S. Constitution

The House of Representatives is established by Article I, Section 2 of the U. S. Constitution

The Senate is established by Article I, Section 3 of the U. S. Consitution as amended by Amendment 17, which changed the manner in which Senators are elected.

The Executive branch is established by Article II of the U. S. Constitution 

The Judicial branch is established by Article III of the U. S. Constitution.

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The presidential oath of office is prescribed in Article II, Section 1, final paragraph of the U. S. Constitution.

The Constitution doesn't explicitly mention executive orders, but their legal basis stems from the President's powers under Article II and from delegations of power by Congress.

Amendment 1 is part of the Bill of Rights.