Does obstructive sleep apnea usually cause many symptoms?

Not necessarily: The number one daytime symptom of obstructive sleep apnea (osa) is sleepiness. Turns out that only about 20% of osa patients complain of sleepiness. Thus, the majoriy of osa patients may not subjectively perceive daytime symptoms. Overall, osa may be a sneaky creature which makes it imperative to actively seek for symptoms/predisposing factors.

Yes: Besides the daytime somnolence and lethargy, osa has been shown to increase risk of hypertension, stroke, and other cardiovasular disease. These issues in turn casue severe symptoms. These issues are detrimental to overall health and decrease lifespan.

Yes: Yes. A list here: http://www.mayoclinic.org/diseases-conditions/sleep-apnea/basics/symptoms/con-20020286