FTSE 250 telecoms firm TalkTalk could be in line for around £60m of compensation from larger rival BT.
Shares in TalkTalk jumped as the markets opened, subsequently falling back and current trading nearly 0.5 per cent up on Friday's closing price.
The fine was discounted by 30 per cent after BT said it will pay back what it owes to communication providers such as TalkTalk, Vodafone and Sky.
BT estimated the total compensation it will need to set aside is £300m.
The fine was announced overnight, catching many telecoms firms on the hop, as historically Ofcom makes penalty announcements on or around 7am.
And while TalkTalk said it didn't know exactly how much of the £300m it is in line to receive, a spokesperson for the firm said it is confident it "will be in the millions".
TalkTalk has around 15-20 per cent market share of broadband affected, meaning it could receive £60m if compensation is divided up on this basis.
Meanwhile, Vodafone, which first alerted Ofcom to Openreach's failings, welcomed the watchdog's decision "to hold BT and BT Openreach to account".
A spokesperson for Vodafone said:
We hope this ruling will encourage BT Openreach to finally drop the unacceptable practices it has used to avoid paying compensation for late delivery of fixed fibre lines, which have impacted businesses across the country as well as our own 4G roll out.
"We look forward to improved quality of service from Openreach in order to deliver high speed broadband to businesses up and down the country."