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                                                                     EXHIBIT 5.1

                    [Letterhead of Willkie Farr & Gallagher]

August 29, 2001




XO Communications, Inc.
11111 Sunset Hills Road
Reston, VA 20190

Ladies and Gentlemen:

We are delivering this opinion in connection with the Registration Statement on
Form S-3 (the "Registration Statement") initially filed by XO Communications
Inc. (the "Company"), on August 17, 2001, with the Securities and Exchange
Commission under the Securities Act of 1933, as amended (the "Act"), with
respect to (i) $517,500,000 aggregate principal amount at maturity of
convertible subordinate notes due 2009 issued by the Company on January 12, 2001
(collectively, the "Notes"), and (ii) shares of the Company's class A common
stock, $0.02 par value per share (the "Shares"), issuable upon conversion of the
Notes. The Notes have been issued pursuant to an Indenture, dated as of January
12, 2001, between the Company and U.S. Trust Company, National Association, as
trustee (the "Indenture"). The Notes and the Shares are to be offered and sold
by certain selling securityholders of the Company from time to time as set forth
in the Registration Statement, and any amendments or supplements thereto.

We have examined and are familiar with originals or copies, certified or
otherwise identified to our satisfaction, of such corporate records,
certificates of public officials and such other documents relating to the
incorporation of the Company and to the authorization and issuance of the Notes
and the Shares, including the Indenture and Registration Statement, and have
made such investigations of law, as we have deemed necessary and advisable. In
such examination, we have assumed the genuineness of all signatures, the
authenticity of all documents submitted to us as originals and the conformity to
authentic originals of all documents submitted to us as copies. In rendering the
opinions expressed below, as to questions of fact material to such opinions we
have relied upon certificates of public officials.

Based upon the foregoing and subject to the qualifications and limitations
stated herein, we are of the opinion that:

1.      The Company is validly existing under the laws of the State of Delaware;

2.      The Notes have been duly authorized and are valid and binding
        obligations of the Company, enforceable against the Company in
        accordance with their terms, except to the



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        extent enforcement thereof might be limited by (i) bankruptcy,
        insolvency, reorganization, moratorium, fraudulent conveyance or other
        similar laws affecting creditors' rights generally, and (ii) general
        principals of equity regardless of whether enforceability is considered
        in a proceeding at law or equity; and

3.      The Shares issuable upon conversion of the Notes have been duly
        authorized and reserved and, when delivered upon such conversion in
        accordance with the terms of the Notes and the Indenture, will be
        validly issued, fully paid and non-assessable.

We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement referred to above and to the reference to our firm under
the caption "Legal Matters" in the prospectus included in the Registration
Statement. We do not admit by giving this consent that we are in the category of
persons whose consent is required under Section 7 of the Act.

In rendering the foregoing opinions, we do not express an opinion concerning any
laws other than the laws of the State of New York, the Delaware General
Corporation Law, the applicable provisions of the Delaware Constitution and the
federal laws of the United States of America.


Very truly yours,

/s/ Willkie Farr & Gallagher