Pros
Everyone I worked with was very nice and helpful. It definitely feels like a tech company, which makes sense, given that so many of them came out of the game industry, with no one fitting Wall Street stereotypes.
Even though their background was tech, they've been at this for 10+ years. They have a deep understanding of their customers needs and prioritize projects well.
They also don't micromanage you. You have your work that needs to be done, and you have the freedom and responsibility to manage your own time and get it done, with plenty of people willing to help out on Slack if you hit an issue.
Cons
Within finance, I've worked more on building trading application, while oneZero builds the infrastructure. For me, it's more exciting to work on trading systems, but that's personal preference.
Some parts of the code base are getting a bit unwieldy, in line with normally happens for a project of this age. This is mitigated by many long-term employees still working there and being more than willing to help you navigate some of the denser areas. Also, I was given the opportunity to work on several sizeable projects where I got to write mostly new code.