An abbreviation for "Original Video" which is the Japanese term for "straight to video".
These videos are usually created with a very specific core consumer base in mind since it's not for broadcast TV or theaters.
Sentai has a few examples of this. Like the Versus series was originally an OV before they started showing the movies in theaters. The Inazuma Spark one offs were also OV's.
V-Cinext is a marketing strategy created by Toei. The concept behind it is that a direct-to-video release will receive a limited event release in movie theaters before coming to video.