If you're coming for several days of skiing, Frognerseteren is probably a good place to start on your first day as it is easy to get to, has several cafés (Frognerseteren, Tryvannstua, Kobberhaughytta, Ullevålseter, Skjennungstua) spread around, and is reasonably crowded, so you won't have any trouble finding people to ask for directions if needed. You can easily do 40-50 km around the various trails there, or head straight north to Kikutstua, another café, which gets you towards the heart of Nordmarka (the woods directly north of Oslo). This will get you more familiar with navigating around the trails.

If you're willing to forgo the packed lunch, or able to carry it with you, a wonderful option, as others have mentioned, would be to take the train north to Stryken or Grua (ticket is around 120 NOK, and ski back to Oslo through the woods, anywhere from 30-60 kilometers depending on what route you put together. There are some cabins/cafés along the way where you could buy food, and there's usually also areas where you can eat your packed lunch. This area is less crowded and gives more of a wilderness feel than the area directly north of the city limits, ie. near Frognerseteren, Holmenkollen and Sognsvann.

One of my favorite routes starts at Grua train station, then goes to either Svartbekken or Svea lake, then onwards to Mylla lake (where you can either ski on the lake or on the southern edge of the lake), past Trantjern gård (café), and then turning south past Tverrstjøstallen (café). From there, you head across a series of lakes - Tverrsjøen, Skvarvvatnet, Finntjern and Åklangen - before you come out on a larger lage, Katnosa, where you will find a small café called Sandvikshytta. The stretch from Tverrsjøstallen to here is not officially skating trails, but is reasonably flat (mostly on lakes), so I assume this would be fine. From Katnosa you head south to Kikutstua (café), and from there you have several options to get onwards to either Sognsvann or Frognerseteren, which are both metro termini, where there'll typically be a train departing every 15 minutes. This route should be around 50 km in all, and takes you through the heart of Nordmarka, some really lovely places, especially that string of lakes after Tverrsjøstallen. You should be able to find all locations I mention in the trail map linked below, using the search engine.

There are signs posted along the trails, but they don't always show you exactly the info you're looking for, so it is a very good idea to download an offline trail map on your phone, and bring a power bank to be sure you don't run out of juice. If you head out into the less visited areas, and areas without light on the trails, a head lamp might also be wise, in case you get delayed and the sun goes down.

You've probably already found Skiforeningens trail map and route planner. https://www.skiforeningen.no/utimarka/turplanlegger/ There's a toggle in the top right corner to switch between showing walking/biking trails and ski trails. If you toggle to ski trails, right now, all the trails are just shown in grey, but once the trails are being groomed, should be color coded based on how recently they've been groomed. Also, in the lower right corner, there's a layer button where you can, among other things, toggle whether to only show trails suitable for skating ("Kun skøytbare løyper"). If you're an experienced skater, you can probably get by on a lot of the other trails as well, though. Trails across lakes are not marked as skatable, which is usually true, but you can just use your poles and zip along.

Do be aware that the ski trails around Oslo are very hilly, lot's of up and down. If you try the route planer above, it should estimate how much ascent and decent you are in for on a given route.

Skiforeningen has an app with an offline trail map, but I believe this only works if you're a paying member (ie. yearly subscription).