Out Of Memory Crashes

While there are different reasons for RimWorld to crash, probably the most common reason is just because the system runs out of memory - often while initializing the game (loading mods.) Modded RimWorld can at times use up surprisingly large amounts of memory (vRAM/RAM) which is mostly caused by mod-assets like textures. It is in general recommended to rule this out first, since it is a really simple process, when your game crashes and only engage in a more complicated troubleshoot later if necessary. 

First and foremost you want to free up as much memory on your system before trying around. Ideally reboot your system, make sure there are no Windows updates queued and then run the game without overloading your system with other processes. 

• Reducing mods
  In many cases this will be the most obvious solution. If your system can't handle the amount of textures added by mods, removing mods that add them is just the most reliable method to deal with it. One of the many reasons, bloating your modlist is highly recommended to avoid in the first place. To help identifying which mods add a significant amount of textures you can use Graphics Settings+ - it adds an UI to the main menu/mod-settings that shows an estimation of how much each mod adds to the overall texture-load. So you can easily spot the heaviest hitters and decide yourself if removing them is a viable option.

• Converting mod-textures
  Converting mod-textures (to the .dds format) - as described in the linked guide - can reduce the texture-load as well. The impact differs quite a bit, depending on the actually used mods, but it always will be an improvement. It also removes the need for the game to convert texture-files itself, which is often the process that actually causes the game to crash in these cases - so it additionally 'stabilizes' the startup-process in a way.

• Increasing your systems pagefile/swap/virtual memory
  This process differs vastly depending on your use Operational System. Linked above is a guide for Windows-systems, if you use different ones and don't know about the process yourself, please consult the internet for any guide describing the process for your specific case.

• Increasing RAM
  No, this is not a troll-answer. If every other attempt fails your system-resources are simply not cut-out to handle a modlist like you're trying to run. The simplest way to deal with it really is throwing money at it - at least to a degree. While you can make sizeable modlists run on 8GB RAM systems if you know what you're doing, having less than 16GB in these days can be a huge bottleneck. How much memory any given modlist needs differs vastly - ranging between 2-3GB total to 30-40GB in some extreme cases. An 'average' modlist of 350 mods, not using any extreme outliers, tends to range somewhere from 8 to 12GB though - so having less than 16GB of total memory makes handling those lists more tricky.