Hi- been putting my old Kona together as a single speed. Finished today, but when I press hard on the pedals the chain skips/slips. Not sure where the problem is. There are no tight links but it slips nearly every time I press hard. Any ideas?

Parts fitted are (all new): On One chain tensioner, On One Single speed rear sprocket (16T), Blackspire Mono Veloce chainring (39T), SRAM PC-1 single speed chain.

Chainline looks pretty good...

How much slack is in your chain when you run it as singlespeed?  If you've too much slack, then the chain won't engage on many of the sprocket teeth and may slip, particulary if there's not enough tension.

Note that the On-One "Doofer"-type tensioner won't cope well with more than 1 link of chain slack.  The On-One "Other Doofer" may be ok providing you have it nipped up tight.

I've never had a chain slip on the SS, even when it has been run so loose that it slaps the chainstay... I've never used a chain tensioner though, always a SS specific frame.

Is your freehub slipping?