Ok, so here we are. This is mu build I will be using to get MaxHP and Max Level in Oblivion. I am using Oscuro's Oblivion Overhaul, so I couldn't care less about level scaling.

Max Level seems to be 49 (if you don't exploit jail). It is because you can't choose specialization, which wouldn't give you bonuses to some of major skills, chosen for attributes.

Race: Nord (it seems that Nord is my new favorite race).
Gender: Male.
Specialization: Stealth.
Favorite Attributes: Endurance, Luck (I want 100 in all attributes as well).
Major Skills:
1. Hand to Hand.
2. Block.
3. Athletics.
4. Security.
5. Illusion.
6. Mysticism.
7. Alteration.

It seems that I will need to level slightly "inefficiently" to avoid capping attributes too early, because when attributes are capped, you can't level up anymore.

Ok, according to my calculations, Bosmer can get 1 more Level than any other race, but they will have lower max HP.

Also, I heard somewhere that Level is hard capped at 50, is i true or it is just some misleading rumor? It is important to know, because so far max Level (if you are not playing Bosmer and don't exploit jail) is 52.

Edit: actually Bosmer won't have lower max HP. If they max out Endurance as soon as possible, they will have 8 less HP than other races at Lv. 52, but will gain yet another 10 HP at Level 53, so they will have 2 HP more than other races. That's according to my calculations and IF max Level isn't hard-capped at 50.

Edit #2: I forgot that Nords have bonus Endurance, so Bosmer will have lower max HP. And there is no hard cap on Level. So, as Level itself doesn't matter much in terms of raw power and survivability, Nords look like all around best choice.

One thing to note is that, in Oblivion, your height affects your movement speed, and it is a multiplicative factor.

Male Bosmer is actually pretty bad, getting only 90% movement speed as a result of being the shortest race/class combination.

Female Bosmer is better in terms of height (and hence movement speed), but has a lower starting Endurance, so you won' get as many hit points.

Altmer get the best movement due to having a 110% height factor; however, you do suffer a bit in Endurance. Male Altmer start with 40 Endurance, which isn't as good as 50 but still decent. Female Altmer start with only 30 Endurance, but have better starting Speed.

(By the way, for speedruns, the choice is male Redguard for any% (Adrenaline Rush gives you speed to speed up the start and clip through walls (with save/load abuse) mre easily) and female Altmer for no OOB (better starting Speed then male, and that 10% height bonus adds up when you're drunk with 500+ speed all the time).)

Also, it looks like Knights of the Nine has an effect that can give you a permanent 5 point Endurance boost (that can be disabled to raise the base higher), and Shivering Isles has an effect that lowers your base Endurance (so you gain fewer HP but can raise base Endurance higher for more HP at later level ups). This, of course, makes truly getting the maximum possible HP more complicated. (Has anyone done an analysis on this?)

Edit: Nords have 106% height, so while not ideal for movement speed, they're at least tied for being the second best race for this purpose. Also, only male Nords get the endurance boost.

If I understood it right, OOO (anti-level scaling mod) doesn't apply to Shivering Isles, so for those who hate level-scaling, it might be better to wait going to Isles before max level.

I also heard about some exploit in Shivering Isles (probably you speak about it), which allows to effectively reach Lv. 155 or 156 if you use it together with jail. UESP had info on it (I lost the link, but I did read it just yesterday).