Curse of Osiris increases the character level up to 25 from 20.
Curse of Osiris now requires players to have a power level of 330 in order to access the prestige raids and firestrikes from the base game.
If you're new to Destiny 2 then you might surprised to learn that since the launch of the Curse of Osiris expansion, you're now locked out of some high-level activities if you're playing without the new DLC. Both have been raised to a 330 recommended Power Level.
One video in particular that's been making the rounds online (from YouTube user SuperBrunch) highlights the weapon's over-effectiveness as the player takes down 6 players in just 30 seconds using Prometheus Lens.
In addition, you'll also need the DLC to access the heroic strike playlist, which returns to Destiny 2 as part of the DLC. One of the achievements for Destiny 2, The Prestige, requires the player to either complete the Leviathan raid or the Nightfall strike on the prestige difficulty. Now, the Leviathan raid and the Nightfall and Trials of the Nine firestrikes require the player to have a power level of 330 before they can attempt these events. Although players without Curse of Osiris can play them, they are much more hard than they were before the launch of the new expansion.
It will be interesting to see if there are any Sony policies that will require Bungie to make changes, specifically regarding the locking of a Platinum trophy behind a paywall outside of the vanilla game purchase.