Not tested it but it should at least compile ;)
void CalcAABBFromOBB(Microsoft.DirectX.Vector3[] obb, Microsoft.DirectX.Vector3 minB,Microsoft.DirectX.Vector3 maxB)
{
System.Diagnostics.Debug.Assert(obb != null);
minB.X =maxB.X = obb[0].X;
minB.Y =maxB.Y = obb[0].Y;
minB.Z =maxB.Z = obb[0].Z;
for (int i=1;i<8;i++)
{
if (obb[i].X < minB.X) minB.X=obb[i].X;
if (obb[i].X > maxB.X) maxB.X=obb[i].X;
if (obb[i].Y < minB.Y) minB.Y=obb[i].Y;
if (obb[i].Y > maxB.Y) maxB.Y=obb[i].Y;
if (obb[i].Z < minB.Z) minB.Z=obb[i].Z;
if (obb[i].Z > maxB.Z) maxB.Z=obb[i].Z;
}
}