Sorry, looks like for some reason you can't buy OSL-LIS via HEL - I was looking the other way round (LIS-OSL via HEL) which does exist. However here's an example link for ARN-LIS via HEL at £582 with a 1-night stay, plus a return to various UK/Irish airports from £250. Outward positioning flight to ARN starts at £23 if you're willing to brave FR or £60-70 (or similar if you do a redemption, assuming a value of 1p/Avios) on BA.

If you wanted to avoid a night in LIS you could buy two one-way flights and do a back-to-back (out, return), though that costs slightly more at £614.

I'm seeing plenty of dates where these prices are available:



HTH