What do OVF and EVF mean in SLR and single camera?

Ovf refers to the optical viewfinder of a SLR camera. It directly looks at the light reflected by the lens with the naked eye. The advantage is that there is no delay, but it may be darker in the case of insufficient light.

Evf refers to a single electric electronic viewfinder, which collects the scene in front of the lens through the sensor and then displays it on the LCD screen. This viewfinder is WYSIWYG, but there may be some differences in sensitivity compared with the optical viewfinder.