No two ovaries are alike, and indeed, the same ovary can change its architecture from day to day. This is because ovarian follicles are present in different numbers, positions, and states of maturation throughout reproductive life. All possible developmental states of follicles can be represented at any time, along with follicles that have committed to death (termed follicle atresia). Static histological and whole-mount imaging approaches allow snapshots of what is occurring within ovaries, but our views of dynamic follicle growth and death have been limited to these tools. We present a simple model of the complex mouse ovary, called “ŌvSim”. Simulating the growth and fate of each of the approximately 3000 postpubertal mouse ovarian follicles over time, ŌvSim produces follicle numbers that closely match numbers predicted by histomorphometric and imaging analyses (e.g., follicle counts) and data on ovulatory rates. ŌvSim relies on the specification of three key parameters or inputs to the simulation: the probability that individual primordial follicles can growth activate daily, the fraction of granulosa cells that survive as follicles grow, and the probability that individual follicles can commit to atresia daily. When the probability of daily growth activation is near 0.005, granulosa cell survival is 0.88, and the probability of atresia for all follicles is near 0.1, ŌvSim simulates follicle growth in a fashion that closely matches biological measurements. ŌvSim thus offers a starting platform to simulate mammalian ovaries and to explore factors that might impact follicle development and global organ function.