The space domain is regarded as the only known physical dimension of lightwaves left to be exploited for optical communications. Very recently, much research effort has been devoted to using orbital angular momentum (OAM) spatial modes to increase the transmission capacity in fiber-optic communications. However, long-distance low-crosstalk high-order OAM multiplexing transmission in fiber is quite challenging. Here we design and fabricate a graded-index ring-core fiber to effectively suppress radially high-order modes and greatly separate high-order OAM mode groups. By exploiting high-order OAM mode group multiplexing, together with wavelength-division multiplexing (WDM), i.e., 12.5 Gbaud 8-array quadrature amplitude modulation (8-QAM) signals over OAM4 and OAM5 modes on 112 WDM channels (224 individual channels), we experimentally demonstrate 8.4 Tbit/s data transmission in an 18 km OAM fiber with low crosstalk. Multiple-input multiple-output digital signal processing is not required in the experiment because of the large high-order mode group separation of the OAM fiber. The demonstrations may open a door to find more fiber-optic communication and interconnect applications exploiting high-order OAM modes.