Optical frequency domain reflectometry (OFDR), featuring high spatial resolution and precision, has been regarded as an important technique in the monitoring and diagnosis of fiber-optic links and networks and distributed fiber-optic sensing. The sweep nonlinearity and phase noise of laser are the key issues to be dealt with for the broadband optical linear frequency sweep in OFDR systems. In this work, we report on laser sweep nonlinearity and phase noise control based on delay self-heterodyne optical phase locking, which allows highly linear laser frequency sweep with an 8 GHz sweep range and a 160 GHz/s sweep rate with low phase noise. Experiments on both fiber-optic link monitoring and distributed fiber strain sensing are carried out based on such laser source. In the former experiment, we achieve long-distance OFDR with a dynamic range of 27 dB and a high spatial resolution of 4.3 cm over a 240 km fiber-optic link. In the latter, distributed strain sensing with 5 cm spatial resolution is realized. The experiments verify the effectiveness and superiority of the proposed method in controlling laser frequency sweep errors by phase locking.