OPI or Illegal Participation?

[Zeke5]

Play: (Federation)
3rd and 7 from A's 40. A86 is running a deep sideline pass
route where he, under his own power, steps on the sideline
at A's 43 and continues downfield where he catches a pass
inbounds and is tackled at B's 40.

What penalty do you assess A with? Illegal participation
or offensive pass interference?

I've heard it both ways from the guys in my chapter. I think
they are both too drastic (15 yds), but at least the
participation foul doesn't carry the loss of down.