Why opposite? The arrows point in the same direction: https://imgur.com/lSkukbQ
But really, one-way direction for footways is completely unpractical to map, even when there is some signage that prescribes it. People tend to ignore it completely, and would expect routing software to direct them in the shortest direction, not in the opposite way. Of course, for narrow passages and/or difficult waymarked trails one way tags are indeed useful; but in our case in practice it's oneway=no.