Men with lower sperm counts can absolutely have biological children the odds have never been better in this age of fertility treatments. Depending on the exact sperm count, many men with oligospermia also get their partners pregnant naturally though. The odds go down as the sperm count decreases, but many men get their partner pregnant without even knowing they have a low sperm count, and others who are aware of their oligospermia still manage to conceive naturally too.
Oligospermia can sometimes be cured, too, if underlying conditions are discovered and treated. Simple lifestyle changes like quitting smoking and alcohol, and starting to eat more healthily, are enough for some men who previously had a low sperm count. In cases where treatment is not possible, and where a couple has not been able to get pregnant due to the man's low sperm count (sometimes in combination with female fertility problems as well), there are a few different treatment options.
Where men have a slightly lower sperm count, the simplest assisted reproductive technique intrauterine insemination may give just the extra boost they need. In more extreme cases, there is IVF with ICSI. ICSI stands for Intracytoplasmic Sperm Injection. This means that a single sperm is isolated and injected directly into a single egg in a laboratory. This procedure differs from "normal" IVF where sperm fertilize previously extracted eggs spontaneously once a sperm sample is added to the petri dish with egg cells.