Constant case of the Grothendieck-Serre conjecture in mixed characteristic

Let D be a DVR of mixed characteristic. Let G be a reductive D-group scheme. Then the Grothendieck-Serre conjecture is true for the D-group scheme G and any geometrically regular local D-algebra R. Also we prove a version of Lindel-Ojanguren-Gabber’s geometric presentation lemma in the DVR context.