Tournament organisers European Rugby Cup will investigate events at the Liberty Stadium in Swansea after Tigers chief executive Peter Wheeler and chairman Peter Tom lodged a complaint.
Leicester went out of the competition as a result of their 17-12 defeat, with the Ospreys and Clermont Auvergne securing quarter-final places from Pool 3.
Competition rules suggest the match result must stand, yet the Ospreys still face an anxious wait to discover what punishment – if any – ERC decides to impose.
England were fined 10,000 during the 2003 World Cup when wing Dan Luger featured as a 16th player for 34 seconds of their group-stage win against Samoa in Melbourne. Unlike in Luger's case, it is claimed on this occasion that Ospreys' full-back Lee Byrne was involved in helping to thwart an opposition attack.
Byrne had gone off suffering from a dislocated and bloodied toe, being replaced by centre Sonny Parker, but Byrne was sent back on three minutes later without any of his team-mates going off. Fly-half Dan Biggar then departed after about a minute.
Wheeler said: "We have lodged a complaint because at a significant point of the game the Ospreys had 16 men on the pitch, and that 16th man interfered with, or was involved in, a break by (Tigers scrum-half) Ben Youngs and played a significant role in nothing developing from that."