Let's say there are 2 characters vs a monster.
1 character is unconscious.
If the 2nd character decides to stabilize the first then are they subject to an opportunity attack? Or are attacks against them at Advantage?
Note, I've read this Does stabilizing a character in combat provoke an attack of opportunity? but think this is a slightly different case as the 2nd character is still in combat but trying to heal their fellow party member.
I am going to assume that "you" are the character that is trying to stabilize the other.
The only thing in D&D 5e that provokes an opportunity attack is leaving an enemy's reach.
You can make an opportunity attack when a hostile creature that you can see moves out of your reach. (PHB)
Using your action to stabilize a creature is not movement at all and thus will not provoke an OA.
It does not matter where you are or how many other player characters there are, conscious or otherwise.
Nowhere in the rules for stabilizing a creature does it say anything about attacks being at advantage against that creature. In fact nothing suggests this would be the case.
You can use your action to administer first aid to an unconscious creature and attempt to stabilize it, which requires a successful DC 10 Wisdom (Medicine) check. (PHB)
Conditions that allow an enemy to have advantage are well spelled out in an ability or spell or determined by the DM:
Sometimes a special ability or spell tells you that you have advantage or disadvantage on an ability check, a saving throw, or an attack roll. [...] You usually gain advantage or disadvantage through the use of special abilities, actions, or spells. Inspiration can also give a character advantage... The DM can also decide that circumstances influence a roll in one direction or the other and grant advantage or impose disadvantage as a result. (PHB)
So no abilities that you have used in the example would cause attacks against you to have advantage. The DM however might decide that they do because of the circumstances of the fight, but most like they would tell you if this were the case.
Stabilizing a creature doesn't provoke an attack of opportunity nor does it give an attacker advantage.
You may also administer potions or cast healing spells or use a Paladin's lay-on-hands as a means of stabilising a dying creature without provoking opportunity attacks nor give advantage to attackers.
An attacker that makes a melee attack against the unconscious character (ie. the one being stabilized), however, will gain advantage and if it hits that hit will be considered a critical hit.
Your DM might decide to use a house rule for stabilizing creatures, in which case characters in his game either provokes opportunity attacks or gives advantage to attackers or both. This might prompt the DM's players to favour spells, class abilities or magic items, such as potions for stabilising downed characters instead.
