Ellis starts in the back row while Williams will pack down in the front row alongside Marc Thomas and Matthew Dwyer.
The Ospreys enjoyed a good start to their LV = Cup campaign last Friday night, seeing off Gloucester at the Liberty Stadium.
Head coach Steve Tandy was impressed with a number of performances from his youthful side and will hoping for a repeat across the Severn Bridge.
“Off the back of a good win at the Liberty last weekend, this will be another tough test of what is still a youthful and relatively inexperienced group of players,” he said.
“We’ve changed things about a bit. There are a few more new faces involved on Saturday and it’s going to be interesting to see how they react to coming up against a team from the Aviva that is renowned for their forwards.”
One of the new faces involved in the pack is Sam Williams who starts at lock. He starts a regional game for the first time, as does Sam Davies at fly-half.
Scott Otten will make his Ospreys debut if he comes off the bench.
“What was particularly pleasing last weekend at the Liberty was the resilience the young group showed and the character to bounce back from the real disappointment of going behind almost at the final whistle,” added Tandy.
“To stay calm, to know the processes and to work their way up for that try was very satisfying and you could see what it meant to them.
“That’s the beauty of this competition, it allows young players to test themselves at a higher level and we’ve seen people earn their stripes in the LV and earn an extended run in the team.
“That’s what we want to see from this group. Who is going to put their hand up?”
