The O-band (1260-1360 nm), located near the minimum chromatic dispersion of standard single-mode fiber, is an important transmission window for short-reach data-center interconnects. Its broader use is limited by the availability of scalable multi-wavelength, high-power and low-noise light sources. Here we demonstrate a high-power O-band soliton microcomb architecture combining an 834 GHz self-injection-locked (SIL) Si3N4 microcomb spanning 1050-1650 nm with a single-stage bismuth-doped phosphosilicate fiber amplifier. The system delivers > 0 dBm per carrier for 21 O-band comb lines over a 100 nm bandwidth, exhibiting low-noise operation and a 5 dB variation in amplifier gain. We validate the amplified source in a 25-km coherent WDM transmission experiment using 32-GBd dual-polarisation 64-QAM. Comb carriers spanning the O-band are characterised under simultaneous WDM loading, yielding an aggregate GMI-estimated throughput of 7.23 Tb/s and a post-FEC throughput of 6.94 Tb/s. This approach establishes a practical route towards broadband, high-power O-band microcomb sources for multi-terabit optical interconnects.