I’m not sure where this came from. In fact I had more in FS2004 than when started using FSX. I still don’t get any.

Like I mentioned on another post, I can fly constantly between payware airports, in the PMDG NGX, with ORBX textures, REX and other texts enhancements. And never get an OOM. That’s why I never moved to P3D. I felt I didn’t need to. 
 

What were people doing wrong to get one?

That is a good question as I have never had one either in the 8 years i have been flying, i fly a range of aircraft from Carenado to PMDG over ORBX scenery, having said that i don't have every slider set to maximum ( I use the AVSIM set up guide for my settings).

 

I never had any OOM’s until I got higher resolution monitors when I upgraded my computer. More pixels, more memory? Before the upgrade, never had one OOM or CTD in over 1500 hrs of flying with FSX SE.  A few months before I switched to P3DV5, I noticed with FSX that my memory would go low enough that I though I would get a OOM, but never did. Did, dovetail fix something? Anybody else notice this?

What are people doing wrong, add-ons?  For example, some planes with 4K textures can take up over 500MB of memory. 

I only started getting OOM errors with FSX once I started combining high-end payware airport sceneries with higher-end aircraft. NL2000 was almost guaranteed to kill me, as did ImagineSim KATL combined with the PMDG 737NGX.

Cheers!

While I had FSX-SE as my main sim (i.e., Oct/'19-Oct'/20), I never had a single OOM in my laptop (specs in signature). But maybe there were 2 factors in play:
1) I used FSX-SE, not FSX-Box. I understand the Steam version was more resistant to OOMs.
2) I was very careful with my settings (mostly no shadows), and while I had lots of AI, I didn't install any single addon airport or photo sceneries. Just AFCADs. There were more than enough for me (and they are still so with P3D4 and FS2020).

I had OOMs from 2010 until 2013 (in my sleep I can still hear those telltale bongs starting to sound) when I replaced a Radeon card that came in my off the shelf gaming system with I believe a GTX 560 TI.  OOM's mostly went away.  They came back after a couple of years into a new system I built in 2015 with a GTX 970.  My savior was the FSUIPC auto-save feature.  Built this current system exactly a year ago expressly for MSFS (sans newer GPU).  When I was so disappointed with MSFS I installed but found I could not activate FSX.  I scooped up FSX SE and knock on wood in now 500+ hours I have had only one OOM and I likely caused it by screwing around too much.

Edited August 2, 20214 yr by fppilot

I've started flying long-hauls early this year in the PMDG 777 and ever since I have been super anxious when it comes to FSX's VAS (Virtual Addressing Space=memory). When I fly 10 hour flights from/to the Aerosoft Mega Airport Zurich (which is basically the only thing I do) on Level of detail radius: Large I will almost always get OOM's either randomly later in flight or just before touchdown. What was the gamechanger for me is putting that setting to Small so it doesnt load so much texture in the distance. And I don't even use things like ORBX worldwide textures or REX... but now I am used to exchanging the look of the sim for stability...

I've found that so long as I do not exceed an LOD of 6 then it doesn't matter what third party scenery or aircraft I use - it is fine. I think the GUI maximum of 4.5 was set quite conservatively in FSX:SE and probably could have been higher. But for me anyway, that is the biggest variable and it shows in terms of loading times too (compared to the default 4.5).

I do understand Dove Tail did a fair bit of work with efficiency, performance and memory usage and like others have noted, it is good enough for me not to have to worry about going to P3D.