The Midosuji Line is your spine
Of Osaka's nine subway lines, the Midosuji Line (marked in red on every map) is the one to understand first. It runs almost the full north-south length of the city, linking Shin-Osaka station, Umeda, Namba and Tennoji — which happen to be the four areas where most visitors end up staying or spending their evenings.
Once you can place those four stations on the line in your head, the rest of the network starts to make sense as spokes running off that spine.
Trains vs. subway vs. private railways
Osaka's rail map is a layering of several separate operators: the Osaka Metro subway lines, JR West's loop and rapid services, and private railways like Hankyu, Hanshin, Kintetsu and Nankai that each fan out toward a different suburb or day-trip destination. They're all covered by the same IC card, but they are separate companies, so transferring between them sometimes means exiting one set of gates and entering another.
Tap-and-go with ICOCA
An ICOCA card (or an equivalent IC card loaded onto your phone) removes almost all of this complexity. Top it up at any station machine and tap in, tap out — the fare is calculated automatically regardless of which of the above operators you're using. There's no need to work out ticket prices in advance.
When the Osaka Amazing Pass earns its price
The Osaka Amazing Pass is a 1-day or 2-day pass covering unlimited subway and bus travel plus free entry to a rotating list of major attractions, including Osaka Castle's main tower. It's genuinely good value on a day where you're hopping between two or three paid attractions and moving around a lot — less so on a day mostly spent walking one neighbourhood like Dotonbori, where you won't rack up enough train rides or entry fees to offset the cost.
Walking and cycling
Central districts like Namba, Dotonbori and Shinsekai are dense enough to walk between comfortably, and a lot of the city's best moments — a covered arcade, a side-street food stall — only show up on foot. Shared bike schemes are also widely available for longer hops along the river or between neighbourhoods.
Trains in Osaka run on a strict schedule and stop running around midnight, with the first services resuming around 5am. If you're planning a late night out, check the last train time before you commit.