Are you aware that to summon others you have to be in human form? Choose "reverse hollowing" option at bonfire. You can also Kindle the bonfire to get 5 more estus flasks.

If you dont have humanities left, you can leave the White sign soapstone near the bonfire. Once you have done this others can summon you to fight the boss. Once you beat the boss you will receive a humanity.

People usually summon two people to fight O&S so it's not too difficult to beat them.

If you are in human form and no one is around the bonfire you can still summon Solaire. His summon sign is in big area prior to O&S (in the staircase). Unfortunately though he sometimes gets stuck and does not follow you into battle.

Also when you summon others they cannot use estus flasks. So you have to heal them with your flasks.

Your soul level is also quite low for ornstein and smough. So maybe try grinding for a bit. And use a shield with good Stability. Higher the stability less you lose stamina with each blow.