Olympus is the son of Herakles and Euboea who is one of the daughters of King Thespius. During his stay in the court of King Thespius, Herakles impregnated 49 out of the 50 of Thespius' daughters including Euboea who he lay with one night. The result of that particular encounter was Olympus.
Euboea & Herakles
by Euboea he [Herakles] had Olympus -Apollod. 2.7.8