The assumption implies that the natural embedding induces an isomorphism $\Gamma(X,\mathscr O_X)\simeq \Gamma(\overline X,\mathscr O_{\overline X})$. Then this means that  the complement of $X$ has at least codimension $2$. 

In addition assume that $X$ is noetherian and $S_2$ (for instance normal). 

In this case *if* $\Omega_{\overline X}$ is a reflexive sheaf, then the restriction 
$\Gamma(\overline X,\Omega_{\overline X})\to \Gamma(X,\Omega_{\overline X})=\Gamma(X,\Omega_{X})$ is an isomorphism.

More generally, let $Z:=\overline X\setminus X$. If $\mathrm{depth}_Z\Omega_{\overline X}\geq 2$ then the restriction $\Gamma(\overline X,\Omega_{\overline X})\to \Gamma(X,\Omega_{X})$ is an isomorphism.
This certainly holds if $\Omega_{\overline X}$ is a reflexive sheaf, but obviously it could hold "by accident" even if one of the above conditions fail, so I am not claiming that these are necessary conditions, but at least they sure seem to provide a natural set of conditions under which the required map is an isomorphism.


**EDIT 1** removed intro paragraph about the starting assumption.

**EDIT 2** added "more generally" paragraph.

**EDIT 3** added noetherian assumption. this is probably not necessary but without this one would possibly be more careful about the other conditions.