An oil filling device includes an oil filling funnel, an extendable tube assembly, a direction adjustment assembly, and a coupling assembly. The oil filling funnel is mounted to a first end of the extendable tube assembly. The extendable tube assembly has a second end mounted to the direction adjustment assembly. The direction adjustment assembly is mounted to the coupling assembly that is attachable to an oil filling hole of an engine. The direction adjustment assembly enables adjustment of the direction of the extendable tube assembly and the oil filling funnel with respect to the engine, while the extendable tube assembly allows the oil filling funnel to be selectively moved toward or away from the oil filling hole. Thus, the oil filling device may accommodate various heights and angles of oil filling hole of engine and allows filling of oil into the engine to be conducted in a smoother manner.