Marcus Tavernier - a former Academy player with the Magpies - struck first for the Cherries just after the half-hour mark, but Osula has equalsied as United look to avoid a third successive Premier League defeat.
Jacob Ramsey, Harvey Barnes and Anthony Elanga came into the side and two of them were involved in the first opening of the afternoon, in the fifth minute.
Elanga played the ball out to Osula on the right and, after a series of stepovers, he found Barnes but the winger dragged his shot wide.
Midway through the first half, the floodgates momentarily began to open in terms of the weather but not in terms of goalmouth incident, although Tavernier - younger brother of former Magpies defender James - shot into the side-netting under pressure from Tino Livramento.
Then in the 27th minute Rayan side-footed wide from 18 yards after Bournemouth won possession and worked the ball from left to right.
But Rayan set up the opener soon afterwards, getting to the byline and crossing low from the right for Tavernier to bundle home.
Evanilson almost added a second from a similar position, trying and failing at full stretch to divert Alex Scott's ball beyond Aaron Ramsdale.
But United almost levelled in the 43rd minute when Lewis Hall's dangerous free-kick caused problems for Scott, only for Đorđe Petrović to fly to his left and spare his teammate's blushes.
The fixture was dedicated to celebrating diversity and inclusion, in support of the No Room For Racism and Show Racism the Red Card campaigns, and Wor Flags paid tribute to the club's first 11 black players - Howard Gayle, Tony Cunningham, Mirandinha, Franz Carr, Justin Fashanu, Andrew Cole, Ruel Fox, Les Ferdinand, Shaka Hislop, Tino Asprilla and John Barnes - ahead of kick-off.
Cole was joined by former Magpies youngster Richard Offiong, who now works for Show Racism the Red Card, on the pitch at half time.
Kieran Trippier entered the fray at the break, replacing Hall, with Livramento moving to left back. Newcastle improved in the second half and the home support continue to back their team, with chants of 'Eddie Howe's black and white army.'
The atmosphere further ramped up a notch when Bruno Guimarães came off the bench to make his first appearance since February - and he had a big role in the equaliser.
He attempted to play Osula through but Evanilson's attempt to tackle the skipper saw the ball squirt straight into Osula's path. He finished calmly but his celebrations were curtailed by an assistant referee's flag.
Following a long VAR check, though, it was determined that Evanilson had got the decisive touch and the goal stood.
Osula had also netted at Crystal Palace this weekend and this strike breathed new life into the home side.
**Newcastle United:** Aaron Ramsdale, Lewis Hall (Kieran Trippier 46), Sven Botman, Sandro Tonali (c), Harvey Barnes, Malick Thiaw, William Osula, Anthony Elanga, Tino Livramento (Dan Burn 74), Jacob Ramsey (Bruno Guimarães 62), Lewis Miley
**Subs not used:** Nick Pope, Yoane Wissa, Jacob Murphy, Nick Woltemade, Joe Willock, Alex Murphy