JINAN, China—The walls of the train station in this eastern metropolis are papered with reminders of Apple Inc.’s challenge in China.

The terminal is blanketed with giant green Oppo Electronics Corp. ads showing a young starlet clutching a sleek Chinese phone. Ads like these have swept across much of the country in the past year—they are visible in bus terminals, subways and shopping malls as well—helping make the homegrown Oppo brand a huge hit.

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