Oceans receive freshwater from precipitation and rivers. yet ocean levels do not change very much from these actions. why are ocean levels not greatly affected?

Ocean levels are not greatly affected because when water is deposited or received from the ocean, at the same time water is being evaporated from the ocean to condense in the clouds and them form into precipitation.

Two reasons:

1).  Because oceans are big and deep, and have a lot of water in them,
while by comparison, rivers are narrow, shallow, and carry less water.
It's like using a hundred droppers to pour water into an olympic-size pool.
The level of the water in the pool doesn't change much.

2).  Because the sun shines on oceans, and tons of water evaporates
from them.  That's where the water comes from to make rain and snow,
and to keep the rivers full, so that they can keep flowing into oceans.
It's a cycle.  The same water flows into the oceans from the rivers,
then evaporates from the oceans, fills the rivers, and flows into the
oceans again . . . millions of times.