Disclosed herein are oligonucleotides, such as nucleic acid inhibitor molecules, having a 4′-phosphate analog and methods of using the same, for example, to modulate the expression of a target gene in a cell. The oligonucleotide of the disclosure comprises a 5′-terminal nucleotide represented by Formula III: wherein Ra, Rb, B, X2 and Y are as defined in the specification. The phosphate analogs are bound to the 4′-carbon of the sugar moiety (e.g., a ribose or deoxyribose or analog thereof) of the 5′-terminal nucleotide of an oligonucleotide. Typically, the phosphate analog is an oxymethylphosphonate, where the oxygen atom of the oxymethyl group is bound to the 4′-carbon of the sugar moiety or analog thereof. An illustrative 5′-terminal nucleotide of an oligonucleotide of the disclosure may have the following chemical structure: