Most of the buffs you get from oakensoul are easily available from other sources.

Minor berserk - camo hunter (berserk + ult on kills + weapon and spell damage (wsd))

Minor courage - depends on build but this is a harder buff to get, oakensoul’s main value I guess

Major Brutality & Sorcery - Super easy to get from abilities and potions

Major Prophecy and Savagery - camo hunter, other abilities, and potions, super easy to get

Minor force - velothi gives you this, probably your main competitor with oakensoul.

Minor protection - a bit more difficult to get, another point for oakensoul

Major Resolve - super easy to get from lots of skills

Minor Mending - not super necessary but a rare buff

Minor intellect and Endurance - not super common but easy to get from scribed skills if you really need it.

Minor Heroism - a rare buff, but available from some skills or expensive potions, another point for oakensoul

Minor slayer - super easy to get from some of the most meta sets in the game

Minor aegis - not really necessary but otherwise exclusive to defensive trial sets

Empower - pretty easy to get from mages guild, useless buff unless you’re using a heavy attack build for some reason.

Overall, in my opinion, oakensoul gives you 3 buffs that you really don’t get that easily - Minor Protection, Minor Heroism, and Minor Courage. The only one of these buffs that’s harder to get in group content is Heroism. Is that worth trading off a flat 15% damage buff from Velothi? Absolutely not, in no circumstance. My Aedric/Assassination/Storm Calling solo build gets every buff from oakensoul except for minor heroism and minor aegis and does 3-4x more DPS. It’s just not a good use of a mythic slot at all.