Polyoxymethylene (POM) is an engineering plastic which tends to release formaldehyde under a melt-mixing process. The present work proposes a simple way to control the formaldehyde generation by a reactive blending with bisphenol-A and amine to form in situ benzoxazines via the Mannich reaction. Further thermal treatment leads to partial conversion of benzoxazines (BA-a) to polybenzoxazines (poly(BA-a)) and the POM-poly(BA-a) obtained is under a semi-interpenetrating network. At that time, the BA-a and poly(BA-a) reached the amorphous phase to induce an increase in elongation at break. When POM blends were electrospun, the molecular orientation of POM was induced and this synergistically functions with the poly( BA-a) network, resulting in an increase in tensile strength which never occurs in the bulk material. The present work shows, for the first time, not only reactive blending as a way to control the formaldehyde generation of POM but also an in situ semi-interpenetrating network as an approach to fine tune the properties of POM.