1. Is the MC OP?
At the beginning, no. Turns out having a tremendous amount of mana isn't enough to be OP, and Death Magic, unlike its name implies, has very low offensive ability, with basically no offensive ability in its early stages.
He does get much stronger but even in the latest chapters, there are plenty of people (well, beings) stronger than him. Although none as good at city management.

2. Is this a harem?
Yes harem, with ghouls, vampires and undead so far.

3. Is there snusnu in this novel?
No, but seeing as the protagonist is still only 7 and already almost none of the heroines are virgins, I see no reason why there won't be later on.

-Protag spent his second life being brainwashed since a goddamn baby (reincarnated people regain their memories at 5 years old normally) and literally had no control over his body. He was forced to research a new attribute of magic that only he could do that turned out to be Death Magic.
He later died during an experiment and resurrected himself as an undead, where he finally became free. He immediately killed all the researchers but got killed by the '100 Braves' (the reincarnated kids) upon walking out because he thought they were going to be his friends. In their defense, they thought he was a random mindless undead. Also, the girl he saved married the guy who had a similar name as him and got double reincarnation buffs.

-After reincarnation as a dhampir, he unusually immediately regained his memories, and his mother was in a country that hates anything that isn't a human, elf, or dwarf (his mother being a dark elf, which they don't consider human), so they hunted her down, tortured her and killed her, and tried to find the protagonist but couldn't. Protag is still a baby at this point.
He finds his mother's soul and takes it with him. He figures out the names of the people who participated in his mother's cruel death and gets slight revenge on the town by giving them a decade or so of work to clean up since that's how long he imagines it'll take him to figure out how to revive his mother.

-He creates some weak undead for a while and hunts bandits to strengthen them, where he learns he can avert the no experience curse from the god by gaining experience through what his minions killed.

-While making his way away from the asshole kingdom, he comes across a ghoul village (ghouls are kind of like a half-human half-undead race), and seeing as the protagonist is generally liked by undead creatures naturally, they take a liking to him.
Ghouls, like many other half-human races (centaur, lamia, mermaid) are considered the same as monsters by the country they're in (ghouls are actually ignorantly considered monsters everywhere, but there is one country that accepts the other half-human ones), but they're actually just a different, slightly more primitive culture. He manages to find a solution for their difficult pregnancy problem, too.

-In the area the ghouls live in, the protagonist learns there's a group of rising orcs enslaving goblins and kobolds and adding them to their forces, going around destroying other ghoul villages and taking their women for the obvious thing orcs do. The ghouls appoint protag as their Ghoul King and he goes around gathering the untouched villages and uses their combined forces to get rid of the menace, rescuing the captured ghoul women.

-However, the humans also learned of this orc force and had been gathering adventurers to deal with it, and the protagonist knows that since there's no orcs, it'll just be switched to killing the stronger ghoul force now. And since trying to hold their ground is meaningless because even if they win, the human side will just gather more forces, he decides to take his ghouls and move across some mountains that would normally be be difficult without the Protagonist's Golem Magic (kind of like an offshoot of Death Magic).

-There, they come across a kingdom ruined by the same asshole country they ran away from, inhabited by undead version of the original inhabitants, giants. They're called giants, but they range from 250-300 centimeters tall. It turns out that the goddess who made the half-human races like ghouls had sent one of the only few sane undead a prophecy of the protagonist's coming, so they got accepted in very easily. The protagonist's presence also more or less brought the other undead giants back to sanity, since undead in this world generally aren't very intelligent.

-He spends a bit rebuilding the ruined city and getting everything set up. He also manages to make some new races using his Death Magic (by accident, really), which are much more humanlike, smarter variations of orcs, kobolds, and goblins. (If you must know, the raped ghoul women decided to keep the kids because it could be useful. Ghoul women are strong in weird ways. While they don't like being raped either, they got over the experience pretty easily.) Eventually, they get attacked by 6000 asshole country's elite forces, along with two of his revenge targets that helped kill his mother. There's also 30 vampires mixed in (a group of 3 purebreed vampires were the ones who arranged his father's death, not that he ever got to meet the guy, and simply go around killing dhampirs because they find them disgusting), sent by purebreed vampires that were also the puppeteers behind the entire army's attack in the first place.

-Protag's forces fend them off, utilizing a strong, high mutational and contagious disease that won't affect his own forces. After he kills them, he resurrects them as undead, and sends them back home as a distraction so that he can steal some crops. Afterward, he destroys the pass that leads to his kingdom so that can't get to it even if they want to. Then he enjoys more city management for a while.

-One of the reincarnated students dies to another reincarnated student and accepts the reincarnation god's request to kill the protagonist. He makes a deal with the god to get a fourth reincarnation. He's a complete scumbag.

-And the current arc is going to a neighboring dukedom, the same alliance of countries that's supposed to support the half-human races like lamia and whatnot. There he meets the 3 remaining revenge targets that helped kill his mother. Their leader is your typical good guy hero who realizes the error of his ways after killing the protagonist's mother unjustly. However, the protagonist is unable to kill him (he doesn't even try) as it turns out he's managed to reach S-rank level of strength (basically as high as it goes), and the protagonist flips out and runs away.

-He's also planning to rescue the living giants that fled from his kingdom when it fell, as it turns out they were taken advantage of and reduced to mining slaves.

And that's a very rough summary of what's happened, keeping to only vague highlights.

Death Magic is kind of like support magic.
At level 1, it can do things like kill bacteria. Yeah. Useful for not getting sick at least, right? And see spirits.

Other abilities it can do is create undead, but these undead aren't particularly strong without a great body to resurrect and a matching soul for it. So early on he resurrects insects that can literally only scout for him, and a skeleton and some zombie animals that could get killed by your average farmer with just a hoe.

Later on he gets abilities like make it where food won't rot, 'kill smell,' kill presence for hiding, can temporarily prevent something from regeneration/healing with a no heal curse, kill insect, kill poison, etc.
One of the more useful ones is being able to detect how deadly something is to him to prevent picking a fight with it, etc. As well as make golems and detect life.

Later on he can restore youth to things/make them age (using this on people won't change their outside appearance, but will make their bones brittle or stronger, etc), make something rot faster.

It also has defensive things, like Deprive Heat which makes hot things or fire not so firey hot, two varieties of Deprive Energy Barriers which negates physical or mana attacks that hit the barrier, although at a fairly high mana cost and people with strong enough attacks can pierce through it anyway.

Honestly, the only method of attacks is another form of magic called Attributeless Magic, which is things like Magic Missile or Levitate. However, he can enchant Magic Missile with Death Magic to increase the damage and basically ignore armor, which so far has really been Death Magic's only direct form of attack. And it's not even solely Death Magic's, so you can see how it has low offense. Otherwise it's poisons or diseases.

Otherwise there's just the element magics like fire and ice, as well as time magic, space magic, etc. But the protagonist is incapable of using those.

There's still other ways to use Death Magic well, but going over LITERALLY all of it is too much for me.