"nearly circular" is correct and usable in written English.
You can use it when describing something that is close to or nearly shaped like a circle, but not exactly a circle. For example, "The top of the table was nearly circular."
Numerous isolated, nearly circular groups of mountains rise above the ridges.
The whole portrait is encased in a large, nearly circular frame, a little off-center on the bill.
Located in the western section of the island, this bright white, nearly circular feature is easily seen from space.
The nearly circular Loews hairpin is the slowest corner of the season, run at just 47 kilometers per hour.
These vagabonds of the sky fascinated 18th century astronomers because of their elliptical orbits, rather than the nearly circular orbits that the planets move upon.
Her shell was nearly circular, almost exactly the size of a manhole cover, with an unexpected concavity on her left side.
The nearly circular island is dominated by Mount Waialeale, rising to 5,243 feet (1,598 metres) at the island's centre.
The orbits of the inner eight moons have low eccentricities and low inclinations; i.e., the orbits are all nearly circular and in the plane of the planet's equator.
Narrow at the northern end, between towering cliffs, the lake widens gradually southward into a nearly circular basin, with rich vegetation on the southern and western shores.
Hungaria asteroids have nearly circular (low-eccentricity) orbits but large orbital inclinations to Earth's orbit and the general plane of the solar system.
Because Neptune orbits in a nearly circular path at 30.1 AU, Pluto is for a small part of each revolution actually closer to the Sun than is Neptune.
