When you see host dashboard, you have Events on right. If there was any event like restart since you've installed oneagent, and you will cover this timeframe using global time picker on top, you will see it there. You should see uptime as well on host dashboard.
You've pasted this question on appmon forum, but you're talking about OneAgent, so this is info for Dynatrace.
Not on this place, lower. Look on Event dashlet. Expand timeframe to see moment of time when restart should be and you will see it there. Restart is so short that may be not visible in availability.
As well here, you have uptime of host, so you cen determine when was las restart.