It is irrelevant whether the OPM designates the President or members of Congress (or Supreme Court Justices) as essential or not. The Constitution provides clauses which protect their salaries. They may or may not actually receive any checks during the shutdown, but they must not be docked any pay.
Article I, Section 6. Clause 1
The Senators and Representatives shall receive a Compensation for their Services, to be ascertained by Law, and paid out of the Treasury of the United States.
No law varying the compensation for the services of the Senators and Representatives shall take effect until an election of Representatives shall have intervened.
Article II, Section 1, Clause 7
The President shall, at stated Times, receive for his Services, a Compensation, which shall neither be increased nor diminished during the Period for which he shall have been elected...
Article III, Section 1
The judicial Power of the United States, shall be vested in one supreme Court, and in such inferior Courts as the Congress may from time to time ordain and establish. The Judges, both of the supreme and inferior Courts, shall hold their Offices during good Behaviour, and shall, at stated Times, receive for their Services, a Compensation, which shall not be diminished during their Continuance in Office.