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We all know that Oh My Girl have been a fan of IU from a long time ago. Here's some evidence!
So let's start with this legendary video of IU saying that Oh My Girl is the group that stands out the most back in 2017 (when she's promoting Palette)
Not long after that, Jiho and Arin on LieV broadcast talking about that IU's interview and their experience of meeting IU at music shows https://twitter.com/IUteamstarcandy/status/856352778469351424?s=19
Seunghee once again mention that all Oh My Girl's members are fan of IU and express their wish to collaborate with her. https://twitter.com/IUteamstarcandy/status/950638393565708288?s=19
To show you how much Oh My Girl loves IU, here's a compilation of them covering IU's songs. https://twitter.com/iCarlyRusher/status/1256868030925037570?s=19
The growth phase actually started in October 2019 when Seunghee and Hyojung talking about they heard from salon's staff that IU's watching Queendom and said Hyojung is really cute. https://twitter.com/LAPUTA05768928/status/1183421745081925632?s=19
In Nov 2019, Hyojung and Mimi attended IU's Love Poem concert in Busan. Not long after that, Hyojung shared her experience on vlive. https://twitter.com/YOOAJAM/status/1202453945915826176?s=19
During December to April 2020 Oh My Girl either mentioned or covered IU's songs as usual. Here's some of it just for evidence.
The big change happened in May 2020 (Oh My Girl was doing Nonstop promotion). IU suddenly promoted Dolphin on her IGstory.
After that Oh My Girl expressed their gratitute towards IU https://twitter.com/IUteamstarcandy/status/1256935716270956546?s=19
Hyojung sent a video message to IU on Kim Shin Young radio because they're so grateful of her. https://twitter.com/YOOAJAM/status/1257542596919869440?s=19
Nonstop promotion on music shows has ended. Then MimPD uploaded new project of YooA covering IU's eight.
IU posted YooA's cover on her IG story and provided the link to MimPD youtube channel.
IU covered Dolphin on IU's Homebody Signal (context: at that time this vid uploaded Oh My Girl was still on Music Bank half year special)
Not long after that vid uploaded, Hyojung posted IU's cover on her IG story. https://twitter.com/_OMG_tweets/status/1276452742802751489?s=19
Seunghee also suddenly fangirling on fancafe because of IU's cover https://twitter.com/IUteamstarcandy/status/1276459163573350400?s=19
About one week after that, Oh My Girl sent a food truck to support IU at her filming site. https://twitter.com/IUteamstarcandy/status/1279225437575475202?s=19
Hyojung shared the reason why they're sending food truck to IU. https://twitter.com/YOOAJAM/status/1280173679544942595?s=19
About one week after that, Edam ent. suddenly drop a teaser of IU's Homebody Signal with Seunghee and Hyojung as the guest.
https://twitter.com/edam_ent/status/1282872513546948609?s=19
https://twitter.com/edam_ent/status/1282872513546948609?s=19
After kept it for a long time, finally Hyojung told everybody about her excitement by commented on Edam ent. youtube channel. https://twitter.com/_OMG_tweets/status/1283017180808835073?s=19
Seunghee also being fangirling on fancafe and told everyone about her experience filming with IU. https://twitter.com/_OMG_tweets/status/1282896104426663936?s=19
The legendary episode of IU's Homebody Signal with Oh My Girl's Seunghee and Hyojung divided into two parts.
Part 1.
Part 2.
Part 1.
Part 2.
On IU's Homebody Signal part 2,
IU revealed that the lyrics for ‘dlwlrma’ were originally written for Oh My Girl
IU revealed that the lyrics for ‘dlwlrma’ were originally written for Oh My Girl
After being comfortable and feel compatible with Sunghee and Hyojung, IU said to think of her as unofficial or hidden member of Oh My Girl 
Seunghee changed her fancafe profile picture into a photo which IU's looking at her and revealed gifts from IU on vlive broadcast. https://twitter.com/IUteamstarcandy/status/1284822614729347073?s=19
Few days later, Hyojung also talked about IU's gifts to her on vlive broadcast. https://twitter.com/YOOAJAM/status/1285564725003083776?s=19
