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Gaining experience is one of the most rewarding things (from a gamist perspective) that players get to do in the game. In OSR+, there are only ten levels of advancement, so it's important to regulate when and why this happens.
Types of Advancement
You have a number of options to decide when players gain a level.
Your choice should reflect the length of your adventure (will it last 3 sessions, or a whole year of real-life game time?) and how powerful you want them to be during the course of the adventure. While OSR+ heroes start off about as powerful as they will ever be at character creation, incremental improvements to attribute scores, larger MP pools, and stacked weapon tactics called perks require you to develop more challenging scenarios from a purely mechanical perspective.
Milestone Advancement
The core rules suggest that PCs level up when narrative milestones are achieved: they solve the mystery of the town; they defeat the marauders who have been attacking the outpost; they recover the sacred artifact from the dragon's lair. You can make the milestone explicit and known to the party, or you can keep it to yourself. In the latter case, players may perceive that levels are given out by fiat, and that's OK, as long as everyone levels up at the same time and players aren't frustrated by it.
In this context, when a player resolves their PC’s story hooks, the player either retires the PC and makes a new one, or changes their PC’s conflict and flaw and creates new story tags to reflect the PC's new goals. You can also reset the PC's fate points and allow the player to choose a different kit to reflect this change.
Story Hook Advancement
In this advancement option, PCs level up when they complete a story hook that resolves their conflict. If a PC's conflict is The Hunted and she defeats the people hunting her during the course of the adventure, she would be entitled to level up and then pick a new conflict and flaw to create new story tags. This is why it's critical that when you design the adventure, you incorporate players' story hooks into the adventure. You can optionally allow a player to change kits if their PC's change in motivation reflects a change in adventuring life.
If you follow this approach to advancement, players will not be able to level up in concert because when they do depends on when they complete a story hook central to their PC's motivations. This may result in some players getting frustrated that other players are more powerful than them, even though advancement in OSR+ is incremental.
Session-Based Advancement
If you're designing an adventure that will only last a specific number of sessions, you can award levels every session or every other session, depending on the cadence of play. This will result in accelerating the adventure's stakes, and necessitate that you scale up the difficulty of your encounters more frequently than you would with other methods of advancement.
No Advancement
Games like one-shots or shorts (which only last a few sessions) may not necessitate leveling up at all. We have run many shorts that resulted in nobody gaining a level, or only a single level being awarded during the course of play, where the short was 6 to 8 sessions long. This is normal, given that by design, all powers and abilities are available to all PCs at level 1.
Alternatively, if you want the party to face challenges that require greater ability, you could start PCs off at a higher level and have them choose perks at character creation.
Exceeding Level 10
OSR+ was not designed to produce PCs with perks beyond level 10, though it's technically possible to create such PCs by simply continuing to scale up perks.
It's not recommended to do this, however, because at a certain point, most unopposed checks PCs make will become trivial, with the only chance of failure being critical failure. Similarly, such PCs can only really be challenged in an opposed roll if the opponent is nearly as powerful as they are.
Changing Kits
You may offer the option for a PC to change their kit if the narrative warrants it, or in lieu of gaining a level at the completion of a story hook (which is useful if you are not practicing story hook advancement). If a character changes kits, they lose the abilities of the original kit and adopt the abilities of the new kit.
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