I dont defend the oil fields usually, My main starts around rawa ruska Hex near Lemberg, to use the river crossing penalty north of Lemberg to my Advantage. Stanislau is covered at first, but usually i fall back from Chernowitz, and even Kolomea after some time. You need to counterattack with brain in galicia, and sometimes i leave Lemberg to form around Przemyśl. Cities like Krakow can be hard to protect, but i rely on pressure from germans taking Kutno and Lodz to discourage Russians from attacking here.
Russia has to be tied up in Caucasus by offensive ottoman positioning, and at some point you need to push towards Warsaw from Austrian and German side. Or make a deep push towards Riga. Overall, Russia has to thinly spread and you need to actively attack it after initial defense, so that it doesnt get to concentrate.
In serbia, i ignore belgrade usually, although it works quite well to take it early. If you take Valjevo, and take Uzice, Serbs will be too spread out to defend and fall on their own. Theoretically, you could also try to push through Cetinje, but this city cant be bombarded from sea easily so its pretty hard. You need to take the diplomatic game into account, as Greece/Italy/Romania joining can bolster their defence. Bulgaria is yours to use, of course.
The key to playing Austria, is managing their navy. You get only some subs, destroyer and a dreadnought, so losses must be managed carefully. If you send Goeben to Pola instead of Constantinople, you can try to force a good fight before the italian fleet with 2 dreadnaughts, and 4 pre dreads + 2 CAs (if i remember correctly) joins. Northern italian front is hard to manage, Trento, Bruneck, can fall. You have to Bombard italian army on the coast and defend every inch of land in the eastern part of the italian front, and regularly counterattack in here.
Adriatic must be firmly in your grasp, which is why an early push towards Cetinje is useful. There are many ways to spread italian forces, but you have to preserve your navy use subs to raid only in small parts of meditarenean, in support of the ottomans and just try to damage capital ships at malta/taranto. Meditarenean is as complicated as the Northern sea, but you need to learn it.