Item strategy in One for All

Recommended items are written assuming normal team comps on both sides, including a mix of magical and physical damage. This is usually not the case in OFA. So you often benefit from varying your items from the recommended ones.

Drop the recommended armor item if you're up against 5 mages (except maybe Zhonyas, depending on who it is). Build some defenses if your champion normally needs a tank line to hide behind. A tank team might be able to get away with all 5 building full tank, but it might be better to have everyone build one damage or semi-damage item.

or in particular can be incredibly good if all 5 of the enemy team are the kind of champions who are countered by that item. On the other hand, or can be very strong if all 5 of your team are the kind who can be helped by that item -- but ability to stack them is limited, so be careful not to build too many.

Also, even if your team doesn't technically have any supports, it is sometimes beneficial for some people to start ; those items are pretty good if you're not solo laning and not taking most/all the CS in the lane, which is often true for at least some players (especially if you don't have a jungler). You don't necessarily have to build them into but it might not hurt, vision is important in OFA too.